So to continue with the FB conversation on the fuel tanks here are some photos shared by Igor Perepelitsa, where it is clearly seen that those "boxes on the floor are not the tanks themselves, but just their covers. The fuel tanks were installed from under the tank, and, judging by the hanging brackets, they were semi-hanging in there.
from the inside (two tanks):
The third tank:
Another vehicle:
View from the bottom of the vehicle: The hanging brackets are also visible:
It would be interesting to find a photo of the actual fuel tanks