How do I true a trailer wheel?
I would skip using the truing stand and just true the wheel on the trailer. You can generally clamp a strip of something to the frame to act as an indicator (have it stick out and just brush the rim, kind of like a rim brake pad), so you can more easily zero in on the worst spots. 

Dishing/centering is harder without a stand, but there should no need to dish/center a trailer wheel that accurately in most cases.