I am unsure just what an 'adjustable' hitch would be but, in order to change the hitch to a higher rating you will have to remove the old one, insure the frame or frame extension can handle the higher capacity, and weld in the new one. (This is quite apart from the rating of the engine, brakes etc.)
As an example, the one difference I noted on the various C class vehicles I was checking out at the time was that the Ford chassis and the methods used to extend them most often permitted using a 5000lb capacity hitch whereas the same RV on a Workhorse/GM chassis did not.... The lower capacity hitch was installed for that reason in most of the cases I looked at.
I would at least check with the RV manufacturer (Coachmen) to see why the lower capacity hitch was installed in the first case. (I DOUBT it was only to save a buck or two!!) The new ones (2012) with either chassis seem to have a 5000lb hitch
