Some dealers will go to bat for you to find the best deal on the loan. Some dealers work with specific fly by night operations and will rake you on the interest rake no matter how good your credit is. It's the same as car dealers. Our dealer got us a fair deal through a local credit union.
That said I would not put a deposit down until they can give you a written paper with interest rate and the name of the lien holder. In other words make the deal contingent on a favorable interest rate and reputable lean holder. Be prepared to walk away. However $799 dealer prep for a $20k trailer? Way overboard. Ours was somewhere in the neighborhood of $150, and the PDI was free. I'm betting they are going to charge you for the PDI.
Good advice about looking elsewhere for both the trailer and loan.