You might want to check out your local marine supply store.  I saw a unit
advertised that was designed to fit into a standard size boat window
(porthole?).  The minimum opening size seemed small, about right for a
Vanagon side window or door window.  It was more than $200, probably more
like $300, but it was specifically designed to start and run with less
amperage draw,  which might be a consideration at campgrounds.  That fact
that it is designed to be taken in and out when the boat is docked might
make it easier for you to mount and store on your camper.  Your unit sounds
like a small house window unit?  Is 5000 btu's  overkill?  How many amps
would this size unit draw?
                                                        Jay