Honestly I think that RVs are kind of a worst-case for batter power, and range extender trailers are unlikely because EVs get progressively less efficient as the battery capacity increases (increasing battery capacity increases the weight). When you're starting with something as heavy as an electric RV, attaching even more weight seems unlikely to pay off.

(Now I'm curious if the relationship between range and battery capacity is merely sublinear or if it actually peaks at some point i.e. there is a maximum theoretical range beyond which increasing battery capacity reduces range)