I would go the cash route, it's how I bought my trailer.
It really will go the same way you purchased your last RV's except you will own it when you drive away.
Negotiate the out the door selling price. Let them know during negations that you will bring a cashiers check from the bank on the day of delivery.
Let them know you will sign the final paperwork only after an acceptable Pre-Delivery Inspection (PDI). Don't sign anything additional than a deposit agreement.
If necessary put a minimal deposit down.
Do the PDI with the dealer.
Make them fix anything BEFORE YOU SIGN anything else. It's amazing how fast they can get things repaired when the money is still in your pocket.
Once satisfied do the paperwork. Verify the purchase price on the documentation is correct before signing anything.
Hand them the check.
Shake their hand.
Take the key and drive away.