withdraw the rest from your retirement fundsSorry but really bad advice. The loss of future income plus the tax load/penalty can be very expensive.
Or get a loan from a relation.Even worse advice, unless it's a relation you don't care about.
So my advice is to nearly max out your CCsWouldn't do that either. Too much high cost debt and too much risk if you get in a pickle and need to charge necessities.
I would just keep looking for financing. If you have a credit union at work, try that first. If you are a home owner an equity loan may work. Rates are fairly low, but you do risk your house if you default. This site came up on a search:
https://www.sefinancial.com/rv-trader/?imt=1&utm_campaign=rv+trader&utm_source=rv+trader&utm_medium=Landing+Page&utm_content=rv+trader.
One last thought is too see how much you can get on an unsecured note (be prepared for high interest) and do the rest on a CC or some other way. Good luck and let us know how you make out. There are many on here who have been in your situation.