Removing the foil an only reduce the thermal performance. Radiant barriers work best when there are multiple low-E layers facing each other across an air gap, such as the exterior facer of the iso, and the interior face of the siding. The emissivity of iso without the facer is pretty high, radiating far more heat outward than with the facer. A trapped half inch vertical air space with aluminum on both sides is worth about R4-5 in average performance, but with the aluminum on just one side of the gap you're getting at-best R1.5-R2.
The critical moisture-trapping issue rests on whether there is a poly or foil vapor barrier layer (or vinyl or foil wallpaper) on the interior side of the assembly. If there is, it has to come out if you're putting foil-faced iso on the exterior.
Taping over the stud edges with FSK tape limits the ability of the stud to dry- don't do it. With the air gap on the exterior side it can dry into the great outdoors, and with a semi-permeable finish material on the interior the stud can dry toward the interior as well. The vapor permeance of wood is pretty low- it's it's own vapor-retarder (that's why it takes a year for freshly hewn cord-wood to dry enough to be burnable as fuel.) True vapor barriers cause more moisture problems than they fix, and if you have them, it matters where they are in the stackup, and the climate. In a motor home the climate could be anything from Key West FL heat & humidity to Fairbanks AK coolth, so tread carefully before introducing a true vapor barrier.
If I understand your stackup correctly you have:
interior air | finish wall surface of some sort | fiber insulation | foil faced polyiso | half inch of air | aluminum siding | outdoor air
Or are you simply re-filling a cavity previously insulated with 2.5" of fiber insulation (or an empty cavity) with 2-2.5" of polyiso?