I would put the door on the outside so I have easier access. Possible exception: if having the door to the inside allows you to slide out a slide (temporarily) into the open area at the end, that could be a very handy advantage.
I don't think it makes any significant difference on the life of the RV; after all, one side is going to be a little more exposed regardless, so what you gain on one hand you lose on the other (and vice-versa the other way around). Maybe the excessively diligent would occasionally turn the rig around end for end to keep the UV exposure more or less equal...but not I.
