You have several choices for an RV. Basically, an internal antenna with gain that's made too boost the signal, an external antenna with gain that receives and transmits in every direction, or a directional antenna which listens better and transmits a stronger signal, but it must be aimed. I just picked up a directional antenna to try, but don't have much experience with it yet. I just unscrewed the stock antenna and screwed in the cable to the external antenna. I use a US Robotics card that I purchased locally. http://www.usr.com/support/product-template.asp?prod=5417 The newer "N" wireless cards provide more range, so spending more to get an N card might be beneficial.
I dealt with Sharper Concepts, 2901 Clint Moore Rd., #193, Boca Raton, FL 33487, (561)989-9902, or
[email protected] . They sell items on Ebay at http://search.ebay.com/_W0QQsassZsharperconcepts , and were very good to work with. I bought the Hyper-Link Technologies Model HG2412P antenna that's about 4" square. Be sure you buy the pigtail connector from them, as I couldn't find one locally and had to place a second order. They had items to me in PA from FL by FedEx in about 2 days. They have a number of different models to choose from.
Good luck!