Tom Medley slumped against the tailgate of the Jeep and stared at the swell of khaki tents.

"You'd think with all the worry and the talk," the 21-year-old Boy Scout troop leader said languidly as the clanking sound of tent stakes being driven into ground disturbed the mid-afternoon quiet, "that we were rebuilding New York City."

Twenty miles from Fredericksburg and a sound wave away from Washington, Fort, A. P. Hill, with its low-ceilinged barracks and tank-lined roads, is not usually described as a Fifth Avenue esplanade. But come July 29, what the Army base lacks in flourish and skyline will be more than made up by Boy Scouts, lots of them.

On that day, 24,000 scouts and 6,000 scout staff members will pour into Fort A. P. Hill for the 10th National Scout Jamboree. And while the scouts will be here for only a week, the work in getting ready for this ultimate in scouting events has been going on for nearly three years.

"I never imagined there was this amount of preparation work," Medley said as he took a break from supervising the field work. "When I went to my first jamboree eight years ago, I just assumed the work began with the scouts and that the scouts made everything. I can't believe how naive I was."

Medley was one of the last advance people to begin transforming this barren stretch of Army base into a tent city for 30,000 people, and he started in June. His supervisor, Gene Coe, who is overseeing the entire project, has spent the last three years preparing for the week-long medley, and Army engineers have been planting stakes, paving roads and laying pipelines and electrical wire since late 1979. The Army has lent the 5,000-acre area to the scouts at no cost, and by the close of the jamboree, 800 military personnel will have helped in some way -- preparing the site or working as security guards, medical personnel or in communications during the jamboree.

And it's not cheap. Officials at national Boy Scout headquarters in Dallas say the price tag for the event will top $6 million, even though the Army is donating the land and all its services. Most of the costs, scout officials say, will be paid by the $195 fee each of the 24,000 scouts and 6,000 staff members must pay to attend the event.

"The way I look at it, it's like building a city from scratch. And, everything you have to have in a city, we've got to have here," says chief honcho Coe, reeling off a list of facilities the completed site will have: an outdoor theater with seating for 50,000, a post office, a daily newspaper, 1,000 ten-man tents for staff members, 47 plastic swimming pools to be used as water tanks, an inflatable hospital, a helicopter pad and an intercamp bus system.

And that's just the beginning, the 67-year-old head engineer adds before continuing one of his many daily rounds of the site.

Finishing touches are being put on rows of showers and portable toilets, water lines are in place, the vast electrical system is almost complete, the last telephone hookups are already done, and Fish Hook Lake, on the southwestern edge of the site, has been stocked with 1,000 bass, 3,000 bluegills and 2,000 catfish, a ready-made fisherman's paradise.

But whatever happened to roughing it?

"Can you imagine 24,000 Boy Scouts arriving with no lines drawn dividing camps or other facilities? can you imagine what would happen?. . . Can you?" asks Coe, who is participating in his eighth national jamboree. "Can you imagine the chaos?"

No time for an answer before he paints an Armageddon-like picture of thousands of scouts wandering around with no assigned place to sleep, fighting over boundaries. Regimentation, he explains, is the key to order. When the scouts arrive with their tents they will be immediately assigned to one of 18 subcamps where they will pitch their own shelters. Pitch first, eat second.

"Now look over here," Coe commands. "There's the rubber rafting. Now on the other side, that's where there'll be a historical treasure hunt . . . and what you see up here is what we call the Merit Badge Midway."

For the visitor who knows little more about Boy Scouts than that they don't sell Girl Scout cookies, Coe explanations begin to do a nervous tap dance on the mind. Maybe it's the heat, spinning up around the 90s. Merit Badge Midway?

That, Coe explains slowly, noting there are no visual props to back up this one -- the midway arrives with the scouts -- will be a series of booths where they can earn merit badges for learning various skills, the stuff of which all good scouts are made, and the key to advancement to higher scouting ranks. The visitor, thinking she has got the swing of it, points to a group of large, eight-foot-square aluminum boxes and asks the obvious. Toilets? No, Coe reprimands, refrigerators. Army refrigerators, designed to stock the 15,252 chickens, 247,040 gallons of milk, 186,000 eggs, and innumerable bushels of fruit and vegetables which will be doled out to the boys so they can concoct meals over their charcoal grills.

After hearing about the Army for at least the 47th time, the question pops. What about the Army? How much is it costing the Boy Scouts? Or, how much is it costing the Army?

"Nothing, nothing. It's a training opportunity these men couldn't get anywhere else. They're learning how to build a city. . . . The Army has to pay these men anyway, regardless if they're working here. They love it. Really love it," Coe says as he and the visitor drive past a group of soldiers driving still more tent stakes into the ground.

"Love it?" ask Pfc. Kerry Knudsen, an Army surveyor assigned to the camp. "You must be joking. You know what we've been doing since we got here? You know what we've been doing every single day for the last year? We've set that (the surveying equipment) in a straight line, walked out 108 feet, pounded in a peg, taken a 90-degree turn, turned around and walked back.

"And we do that 99 times in a day. The only good thing about this assignment is getting a suntan."

But Col. Warren Crofoot, the Army's supervisor on the job, says the jamboree will be a great time for the Army to show its polish and maybe snag a few recruits.

"It will be a perfect opportunity for the Army to show itself in its various facets," Crofoot says. "There will be no active recruitment, but out of the 30,000 scouts there are bound to be a couple of thousand who might be interested in joining the Army."

Crofoot adds that Coe is correct when he says the jamboree is not costing the Army extra money. The men working on the project, he reiterates, would still be paid, whether they were working at the scout camp or on another detail. And to compute the salaries of the 800 men who will eventually work at the jamboree would be nearly impossible -- some have worked on the project three years, some have worked two weeks, some have worked one day. Crofoot was called up from Homestead, Fla., in April to work on the project.

"It will be a great week for the Army," Crofoot says.

And a great week for Coe. "it's the kids. The excitement.If you could see 30,000 kids set up camp you'd know why we do it."

Knudsen, however, doesn't plan on staying around for the opening ceremonies. The day the jamboree begins is the day he leaves town.

"If I had to crawl out of here," the disgruntled private said, "I would do it."