You are approaching it right, establish budget then go on the hunt. Keep little flexibility on hand, in case you find this "irresistible bargain" for little more.
The depreciation on RVs is horrible, that only proves that they were not worth the sales price when new in the first place. I did it once but it didn't affect me that much because I kept it beyond the last bank payment (10 years).
Before and after I bought used and depreciated, one I stole at a bank auction (1 year old for 25% of the original sale price).
How far your budget will go will depend on several factors. The age of the rig, the original price, the make of the rig, the reputation of the maker.
Similar to automobiles, if you want to pend $10K on a used car, you can get almost new Focus, significantly older Cadillac and a much older Mercedes.
Although not always true, higher end products typically have higher quality built into them from the start and should age "more gracefully". Also certain brands are well respected even with many no longer made, such as Teton, Travel Supreme, Alpha, New Horizon, SpaceCraft, Carriage, these tend more towards high end and you might have to get and older unit (for your budget), but you might be a really "happy camper" with one of these.
I bought this one 10 year old, it just showed up on the internet.
One of my clients found this rig last year on the internet.
It was a 10 years old Spacecraft, hardly used by this senior couple, 47 feet long, two bedrooms, sale price $20,000 !!!!!!!!!!!