If you're thinking about South Padre Island for the winter, I'll warn you that in the past, the three RV parks on the island completely filled up after Christmas. Well, maybe not the KOA, but it's really expensive, and if it's not completely booked, it's pretty darn full.
I posted details and photos on SPI upthread. Since Andy Bowie doesn't take reservations, if you get a spot there you can keep it and won't be evicted by someone with reservations. Getting there by about December 20 would give you a very good chance of getting a spot, and after that, it depends on whether someone is leaving--there were two or three that seemed to be used by short-timers once the place was full, but that's about it.
And if you can't get a space in the two weeks before Christmas, you can stay in a motel for less than the RV space would cost you anyway. Seriously--like $25/night on weekdays. But after Christmas, everything changes.
There are some trailer parks in Port Isabel, the closest mainland town, but they stay pretty full, too. They tend to be very cramped, and kind of depressing if you ask me. But very cheap (probably about half the cost of the two county parks on the island). Several of the windsurfers I met at the North Flats sailing location have trailers in Port Isabel and make the drive over the bridge every day, and have done that for several years running.
There are tons of RV parks in the Rio Grande Valley, which is often referred to to as "The Valley" or abbreviated in posts as RGV. The majority of them are geared for winter Texans and many are restricted to 55+ -- it's that kind of place. I wouldn't stay there if my main reason for being there was sailing--it's just too long of a drive.
You won't find anything in the form of mountainbiking, although we rode some singletrack in a little park in Brownsville. Combined with a stop at Rudy's BBQ, it made for a satisfying day trip.