Underground is better.
2 ft under, with vertical rise (preferably in conduit) where it ends near the trailer. You can do it yourself, or hire some low-cost electrician for a piece of mind. You need a conduit only for a rise, not for undeground. One cable all the way, no 2*50 cords.
To determine the cable gauge that you need, run the numbers for anticipated loads (i.e. maximum amps) on 100ft distance. If A/C is not planned, microwave will draw up to 12A, other loads are minor, you total current will be under 30A then.
You might need a junction box near trailer to go from a beefy undeground cable to a thinner wire that you would terminate with RV plug, but this is only if undeground is really thick - I don't remember max diameter that fits RV plug, is it #6 or smaller.
Home Depot have aluminum cable suitable for underground works with gauge up to #0, but you won't need it that beefy.