My advice is that get the trailer the way YOU want it before you make the purchase final. Fill ouot the contract to purchase and make it contingent to whatever specifications you have that are deal killers. Include the words "time is of the essence" and set a dead line for everything to be completed. This gives you an easy way out if they don't follow up on what they agreed to.
The contract is legal because it's contingent on YOU paying them money and THEM providing a specific RV equipped the way you want it. If they don't want to do business that way, you DON'T want to do business with them.