I learned to not give them the opportunity to lie. I go onto a rv lot, equipped with the knowledge I need so that all I'm there for is to look at the trim and floor plans. I know how much I can tow, I know what my GVWR is, and I know where to look for the trailers weights are. If I have a question I whip out my phone and go to the manufacturer's website.
When I'm ready to buy I have my credit union draft me check and I bring it to the office and say this is my check this is all I wil pay, do you want to do business with me?