If you have an open breaker spot in your power distribution panel, you could add another 15 or 20 amp breaker and then wire it to and externally rated receptacle... maybe a twist lock type.
I have a 21' enclosed snowmobile trailer and I had it built with 30 amp service. It has a sub-panel with two 15 amp breakers and about 4 or 5 outlets inside. You could add a sub-panel inside the trailer and then plug into the additional receptacle you add on the back of the motorhome to power the trailer.
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