Everyone runs across them at one time or another. You'll be driving along the highway when suddenly you come upon a long line of trailers, pulled by cars, vans or pickups. Or maybe you'll pass a roadside campsite filled with motor homes, or a campground filled with pop-up campers, set up in a huge circle, wagon-train style.

And they're not just limited to local highways. Hundreds of thousands of miles have been logged by caravaners who have traveled to Alaska, Canada, Mexico and Central America. And come September, a minicaravan is veering off the beaten trail and heading for China. Eleven couples, average age 70 and all members of the Wally Byam Caravan Club, will tow specially built Airstream trailers along a 1,500-mile route along the back roads between Xiamen and Nanjing. Each couple is paying $12,000 to participate in the 28-day goodwill trip.

They are among the 30 million people involved in RVing across the country, according to Gary LaBella of the Recreation Vehicle Industry Association. There are 7 1/2 million RVs on the road now, LaBella says -- a figure that includes motor homes, travel trailers, folding camping trailers, van conversions and truck campers. ("Mobile homes," stresses LaBella, "are not RVs!")

And more and more owners of these vehicles (which cost from $3,000 to $100,000 and up) are joining clubs and traveling en masse, staying in the nation's 13,000-plus public and privately owned campgrounds.

Membership in the nearly three dozen national and brand-name camping clubs, say participants, is turning camping into an extended-family affair, what with group rallies and caravan-style trips.

What's the appeal? Participants list fun, fellowship and adventure among their chief motivations for joining these modern-day caravans. Family togetherness, educational experiences and economic getaways also are cited as major benefits of membership in RV clubs.

There are few young singles in the RV clubs, but young and middle-aged couples and their children are not unusual. The Good Sam (short for Good Samaritan) RV Club now lists more than 450,000 member families, representing more than 1 million people. Good Sam has its headquarters in Agoura, Calif., with 2,000 chapters across the country. Thousands of RVs have shown up at its national rallies, called Samborees.

Rallies, it seems, are a big attraction. The larger ones offer something for everyone -- organized activities and games for kids and, for the adults, square-dancing, potluck dinners or sightseeing.

When the Wally Byam Caravan Club International, limited to owners of those familiar aluminum Airstream trailers, held its 28th annual international rally at Lake Placid, N.Y., earlier this month, 12,000 people and 3,975 trailers showed up -- including the 27-foot Airstream owned by the Cecil Carlson family of Fairfax.

Carlson, his wife, Carolyn, and their 11-year-old son, James, have belonged to the Wally Byam club since inheriting their Airstream five years ago. "This was our first international," says Carolyn, 46, "but we plan on going again next year, when it's in Boise, Idaho."

The Carlsons are members of the Northern Virginia unit, No. 110, and they regularly attend the monthly unit rallies, where anywhere from 15 to 25 trailers (out of 80-85 in the unit) may show up. Their unit, they say, which includes "dentists, PhDs, electrical and mechanical engineers, computer systems engineers" isn't atypical. "The club may be 80-90 percent retired now," adds Cecil, 51, who has a civilian position working in Army personnel, "but we're all doing it for pretty much the same reason . . . for the fun of it and for the fellowship."

Eugene Doyle of Woodbridge, an assistant director of Virginia's Good Sam Chapter, says Good Samers, as they are called, get together for much the same reason. Doyle, 73, who retired after 30 years in the Navy in 1967, and his wife joined the club in 1969 and took their first caravan, with three dozen other RVs, to Mexico that same year.

And while the Doyles have not yet joined the growing number of couples who have traded their homes for a nomadic way of life, they do their fair share of RVing every year. "All my life I wanted a boat," says Eugene Doyle, "but after moving here, I realized you could go up the river and down the river, but pretty soon, you're going to the same places all the time."