When looking at older RVs it's best to look at ones for sale by owner. Request the past repair/maintenance records. By going to his house to look you can get a better idea on how the RV has been taken care of.
Also with an older RV lower your budget by $10,000 and keep that $10,000 in a separate account tagged just for updates/repairs. With any older RV you will have to plan for repairs.
How much do you use a RV? You might be better off just going more recent in years and get another gas. Diesels have a premium and if you don't use it much it's probably not worth it. Perhaps a newer gas or a different one than you have now will be a lot more quiet for you if that's the issue.
Bottom line.... don't go into debt buying a RV.