I think you have the picture now: the answer is definitely "maybe". RVs may come with any of a variety of custom sizes, in either width or length. And some smaller ones may even have unusual corner cuts, depending on the needs of the floor plan. Mattress thickness differs too, but the same is true of home mattresses. Unusually thick or thin mattresses may present difficulty in finding sheets, especially fitted types, to match.
Air type mattresses, e.g. the Sleep Number brand, are also common in RVs, but they are usually in US standard sizes.