We always buy used. It helps to have owned one or two ~ go thru it with a fine toothed comb, try out EVERYTHING, Take a DVD and a CD with you to try out the entertainment systems. Ask them to have the fridge and freezer on, take a thermometer for the fridge and one for the oven. take a long necked lighter so you can check any pilot lights, etc. Look in, under and around everything. Step on just about every square inch of floor ~ you want it solid, if it's not solid, assume there's a water issue and RUN. Look for any signs of discoloration on the ceiling walls and especially on the flooring. Any sign of mold or mildew? Again, not a good sign.
If you locate a decent looking and clean model, do your homework. Look at NADA, Kelly Blue Book and the RV Trader for what they are going for in your area. Once you make your deal, be sure to buy the best warranty you can to cover things like AC units, fridge, etc. which have a way of working then stopping at the most inopportune times.
I always suggest you buy the best quality you can for the money you have to spend. Good luck and happy hunting