Every 50 amp site I've been at also have 30 amp and 15 amp service on the same pedestal. 30's have 30's and 15.
My TT is 50 AMP so I don't have a need for 50 to 30 dogbone, but I do use a 30M to 50FM dogbone when I am at 30 amp sites.
Probably a good idea to have the 50M to 30FM dogbone, just in case the 50 AMP site you go do does NOT also have a collocated 30AMP.
Mike
EDIT: Changed 20 Amp ref to 15 Amp in my post.