TU, actually AU is polyurethane, it is considered a 2 or 3 part, single stage polyurethane. This means you mix at least two parts, base plus catalyst and shoot only one type of paint in several coats.

UH and UF are correct. The cheapest way to go is to use what your local discount paint shop uses. Companies like Eastwood sell single stage products for around $100 per gallon. Depending on how much prep your trailer top needs you could likely find a discount shop that will do a top for about 1 AU (aviation cost unit is $1,000).