Two comments..
First: If you do it yourself clearly label it: 30 amp 120 volt RV outlet.
Second: If you have an electrician install it, go with a 50 amp, Reasons are 2 fold.
1: Every year about this time we read of folks who plugged in to what they thought was a 30 amp outlet and so it was, 30 amps at 240 volts, and they fried all their expensive goodies (Radio, tv, microwave, converter, and possibly more stuff) thousands in damage.
Some had a 30 amp installed by a professional who looked at and hooked it up for 240 volt even after being told it was for an RV even though it SAYS 125 volts max RIGHT ON THE HOUSING. he wired it for 240 volts.
50 amp service they usually get right
Second reason: When you upgrade, that won't take any time at all to upgrade the outlet for your new 50 amp rig.
Then you use a dogbone adapter for the 30 amp trailer.