You are missing depreciation. Loss of income on capital used, except that you will be borrowing and paying interest instead.
I would personally not finance any RV (or any other vehicle or rapidly depreciating asset) for longer than 5-8 years. On a 15-year loan, you will still be upside down after ten years of (mostly interest) payments.
Oil change: Motorcraft FL-820S oil filter is no more than $8, under $4 at Walmart where five quart bottle of the recommended Motorcraft oil is $18. My Ford dealer changes the oil and filter on my motorhome for $19.95, but after the "environmental fee" the cost is about the same. The dealer does a front end and U-joint lube in this package, you need a grease gun for DIY. Dealer changes the oil and filter for my E-350 van for $9.95, no way I can beat that price at home.
Budget for one air filter a year. Depending on conditions, the OEM filter might last 2-4 years, so you could get lucky. Price at the dealer is about $20, I would not mess with substitutes.
Inspection of roof and other seams twice a year is a good idea. I store under shelter, for ten years it has needed inspection only, no re-caulking. My local dealer now charges $180 for a full annual inspection, plus time and materials for repairs. Taking care of the house is much more expensive than taking care of the chassis.