Sorry, sloppy reading on my part. My advice was for new, not used. Used, the only thing you have to go on is the NADA guide which only quotes average and low retail. These numbers may or may not apply to your area. You can also look on Craigslist for your area and see what similar units are listed for. It takes some time and diligence that way because you have watch and see which ones sell and which ones are relisted time and time again. I called my credit union and asked them how they valued an RV for a loan and found that they would use average retail with only a plus or minus for mileage. Private sellers are not retail although that didn't matter to our CU. They allowed no extra for any options. Figure no dealer would pay above low retail for any RV. I used that as my guide when I found an RV I wanted to offer on.
What the current owner owes should have no impact on what you pay for the RV. Why should you assume the loss for their poor bargaining ability? This is one of those instances that you have to let your head rule your heart as much as you can. You also have to weigh finding the perfect RV for you and reflect upon the possibility of losing it if you are too parsimonious. That said, you can always increase an offer but it is real hard to lower it. If you can, try to keep it from becoming adversarial and help the seller work towards making a deal that works for BOTH of you.