I think you need to actually talk to someone, preferably a manager or even the owner, rather than just communicate via email. You will get a better "feel" for the people you are dealing with.
If comfortable, I would offer a substantial REFUNDABLE down payment. Get your financing, insurance, and DMV necessities all arranged. Then make your trip. IF either party is not satisfied, then they can cancel and you get you deposit back.
You are making a substantial effort to make that drive so the dealer should understand you are a serious buyer. If they balk at the refundable deposit, wire it to them or mail it to arrive the day you arrive there, then your commitment making the drive along with money in their hand should be sufficient. In that manner, they are not out anything. The only danger is if someone buys that trailer before they get the money.