can't say I know exactly what your situation looks like, for instance when we cut the opening for our AC we had to keep the outer and inner skins from crushing since there was no ribs at that location to support the unit, so we added aluminum c channel inside then riveted the skins to that which beefed up the opening.
I just did a quick search, would be good to shop around, but PPLMotorhomes have a 14x22 rough opening model and a 18x24 rough opening model available here:
RV Skylight for sale at PPL
I suppose depending on how you do it you could put a patch to resize it, but a bigger skylight might be a better looking repair.