You have 4 choices:
1) Accept the unit as is for the price you agree upon and get different tires yourself after you have possession of the camper.
2) Attempt to negotiate different tires into the deal at the seller's expense. (which probably will not happen, but why not try?) If you don't try, you'll never know.
3) Negotiate for different tires and have the dealer install them, but you pay the dealer for the difference in price.
4) Walk from the entire deal and forget it.
If you really want the camper, you're probably going to have to pay the price. Any modifications you want to make are going to be on your dime. Sorry. It's not really a buyer's market when it comes to RV's. You can negotiate a little, but for the most part, they have to clear a certain amount on their sales too, in order to stay in business. They really do have a "bottom line" they cannot cross over.
