I agree they are heavy, and even more heavy than the bus chassis they are built on top of, but I see that as an engineering problem right? There are bus depot parking structures where unused commercial buses are parked. And my civil engineering friends are fond of saying that with enough steel and concrete you can hold up anything :-). So as a mental exercise in scaling density by going vertical, in my sketchbook using lot "sizes" that were similar to a KOA RV campground as a baseline (about 30' x 60' per space), an 8 story parking complex on a 5 acre lot could hold 250+ spaces.
Ok, and the idea isn't even original, see this: https://99percentinvisible.org/article/mobile-home-skyscrape...