As far as was practical. pull some dimensions off the two trailers and see what and how you can live with this. You did not have any perfect side-on photos of the boat and RV. One of those would be great for visualizations.
If it is for bare RV, then you'd have to add tail lights on it if it went back more than 3', at least in my state.
I can just imagine how it would smooth flow onto the boat.
Add some longitudinal framework from windscreen to transom, and a tight tarp on that. Essentially a boat cover
You could do no more than that.
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2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle
currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears