I've owned saltwater kept boats & done all the maintenance since 1981. Tried almost every product put on the shelves. Watched the technology in waxes & sealants advance over the years. A few things ring true no matter what. Buy the right product for the job. You get what you pay for. If you neglect maintenance, there is no instant easy way to restore the gelcoat.
I treat the trailer like the boats. Use the best fiberglass products available & keep it looking good. This means one waxing/sealing with Mequires Flagship (the black bottle) a year on the sides & a couple times a year on the front cap. Splattered bugs, cleaners & sun put an extra toll on the front cap. We do up to 20,000 miles a year. Alot of bugs.