It's my understanding that pvc can only be used below ground, to make the transition above would require something else. The size and type would somewhat depend on how far it has to run, the elevation change, and what size are you tapping into. Then, determine the max flow you may want in the trailer and if there would be any lawn watering or other big water usages going on simultaneously. There are codes for regular homes as to what size pipe is needed based on the amount and type of fixtures in the dwelling. Trailers sort of go their own way. If this needs a permit (it does in most places), the inspector would dictate the size based on the local codes and practices.