I installed an electric subpanel by CONNECTICUT ELECTRIC with 50/30/20 amp outlets. You would not use the 50 and 30 at same time. You size the copper wire for 50 amps and based on the length of the run from main panel. I went one size up to safely handle 50 plus possible use of 20 at same time. Probably overkill but I can't hurt anything but your wallet. Be sure to use conduit large enough to accommodate all three conductors (L1. L2, Ground). If the path has many 90's and 45's that will make pulling harder too. You probably want 1.5 or 2.0 inch conduit.
I installed three RV electric panels at home, the one described above is convenient to house for RV guest and we use it to prep our unit a few days before a trip; we always start long trips with one or two nights hooked up at home to make sure all systems are GO. That station has water and sewer connections. The other two panels farther from the house are under covered storage for our RV and travel trailer.
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