Wow! That is a long time in storage. Not sure how the custom covers intended for short term storage will hold up.
Your biggest enemy is chafe. An RV has lots of sharp things on the roof & corners. Chafe will damage a cover of lessor quality & the RV too if not covered properly.
I have stored boats, also with lots of corners & sharp objects, for many years. First of all cover those things/areas where the cover will rub.I used multi folded terry towel taped over the corner/sharp object for a much softer turn.
Next is the cover material. For that length of time I would get the best, most durable material to be found. I am thinking along the lines of the covers truckers use. Expensive I am sure, but something you buy once & it will last.