Palmer used to sell an ESP from Cornerstone, and that one got good reviews. I've got no personal knowledge of the one he is offering these days, and haven't seen any information on the site about it either. I'd suggest you talk to Jim Palmer and have him send you a copy of the complete contract, then read that complete contract for all the gotcha's. Every ESP I've ever looked at (I don't buy them, but I'm a curious sort of guy and like to check things out) has had specific information about non-stock items like trailer hitches, as in they will void the coverage. With an ESP, you don't have any federal protections like you do with the factory warranty, so the contract is the final word.
Jerry