Leaving all they Frankenstein references and cost savings of one wire size over another I think it makes sense to install what you want/need. For our trailer at home I ran 12/3 from a 20A breaker in our home panel to a 30A RV outlet alongside our house. That 20A runs everything I would need while the trailer is there. Likewise because of cost and convenience (mainly due to the unweildly size of 50A cords) I installed a second completely separate 30A circuit in our trailer and make a 50A to dual 30A dogbone so now my trailer has basically a 60A service. Doing it that way vs. actually converting the trailer to a normal 50A service left all the original wiring in the trailer there and simply added the additional wiring and outlets along with a 30A service panel for the second 30A service. Doing this also allowed me to use my original 30A detachable cordset and the regular 30A extension cord to feed the 30A lines into the trailer so I didn't have to buy any extra or new cordsets.
Larry