Looks like the OP is looking to finance now, so improving their credit ratings, while excellent long term advice, isn't an option for their current situation. The credit union seems like the best option, since he's current on his auto loan with them. Perhaps that matters, although they are also aware of the debt on the auto, and might not want to take on more risk. Can't hurt to check, won't know unless you ask.
Check online for the rates for people who have good credit, and be somewhat leery of going too far over that. A RV is a depreciating asset, and if you're purchasing it with a low down payment and a high-rate, long term loan, you're going to be underwater for the entire loan period, and pay far more than cash buyers or those who make a large down payment and qualify for a lower, shorter-term loan. It's not a good deal if the only way to make it is on a bad loan. Under those circumstances, better to postpone for a year and improve your credit ratings enough to get a better deal.