While still in the house - And assuming pretty good credit rating -
Take out a loan shortly before you are out of the house, use it to buy your RV, and then pay it off with the proceeds of the house. If the house is used as collateral, or even if it isn't, the interest cost will be minimal.
** On edit - On edit, I think I need to be more clear.
My suggestion is to take out a personal loan (use the house as collateral if need be) just before you sell your house and use the money as a down payment on the new RV. Then pay off the loan after selling the house. The interest for a few months (3-6 ?) will be minimal.