I would call your bank and inquire as to the loan value on your rig. An RV typically has a loan-life longer than a regular car but still, something about 12 years from new is as old as a regular joe is likely to secure financing on. That means a $35k 2003 Model Airstream is probably pushing it and even then loan value would be 80% at the most ($28k) meaning your buyer would need to want the rig badly enough and be well enough off to pull $7,000 from their wallet and borrow the other $28,000 to buy that $35,000 rig. Step back to your rig, a 199x model, and you need a cash poor buyer just to buy what ya got, much less pay for a 4wd conversion. Ages and percentages here may be off, it's been a long time since I borrowed money but the concept is sound.
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Depreciation curves get steep when banks stop loaning due to age, etc.