Many would argue that you should never borrow to buy an RV - they depreciate rapidly and your likely to be underwater soon after you purchase.
RV lose about half of their value in the first 8 years or so and some would argue buying used is better than new because you avoid most of the depreciation and new RV's tend to come with plenty of small/big problems which might be covered by warranty but you might wait weeks for a dealer to get around to fixing them. No fun making RV payments while your rig is sitting on dealer parking lot waiting for warranty repair work.
If it were me - I would buy used - probably a 10 year old rig from owner who kept maintenance records (priceless). I would keep sufficient cash reserve to insure I could keep up with the required maintenance. If you go diesel then make sure you do additional research on the specific engine (some have issues). Also - some rigs have better reps than others - research is key.