Agree with above. The important point is to know exactly what you want, right down to the make and model. Maybe have one more as a back up plan, just in case?
THEN you can start your shopping! At that point, if you find it at a Rally, you'll recognize a good deal IF there is one to be had?
Going to a rally with the idea that you're going to pick out an RV and buy it right then is a plan that's likely going to cost you a lot more than it needs to IMHO.
Most of the time the only thing a rally is good for is looking for ideas....
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake