The hole in the floor allows access to the fuel tank sending unit, which is needed in order to swap fuel tanks. The hole is not there from the factory (as far as I can tell, not 100% sure about this) since the fuel tank on a bus can be mounted in a number of different places, but is cut by the bus mechanics in the course of a tank swap and then patched however (the mechanics on my bus cut the hole halfway through a stiffener on the underside of the floor, which is not ideal). If you have a hole like this in your bus, I believe it means you have had your fuel tank replaced at least once.
In theory you could just weld or screw a patch over the hole, seal it and cover it with your subfloor, but if you ever did need to replace your fuel tank (or access the sending unit for any reason) you would need to cut a new hole. I also read an account by someone some time ago who said they failed an inspection because they had covered over the hole in an inaccessible way. If this hole isn't even a normal thing in a factory bus, I'm not sure how you could fail an inspection for not having one.
I'm going to build a removable patch into my subfloor, although my treatment of the fuel tank access is a little different than most: https://www.skoolie.net/forums/f11/r...tml#post371452
(my fuel tank says 2010 on it, and the bus is a 2003 so it's at least the first replacement).
Maybe somebody reading this can explain why the fuel sending unit on a tank needs to be accessed before the tanks can be swapped. The hoses/cables that run to it are not that tight, so it seems like the tank could be dropped first and then have the hoses disconnected, but that doesn't seem to be the way it's done.