The belly pan can be done a couple of different ways depending the material used. Run it side to side or front to back depending on the area you want to cover. I color coded mine to give you an idea. I have 4 tanks in the belly with access panels to plumbing/electrical inbetween them. Keep in mind I did the belly pan in .032 alclad so I could get 60" wide pieces. Mix and match if you want, who's going to see it??
are belly pan pieces (suggest keeping outside corners smaller since they have compound curves on the edges) There will be long narrow pieces around access panels. Front center can be larger if you don't have the spare tire well.
is a fresh water tank support.
Black are the grey and black water support panels.
are the access panels between the tank compartments.
One other hint. When you do adjacent permanently installed panels where the overlap is side to side, start at the back and work forward. That way the panel on top edge faces to the rear and not the front to invite water intrusion. Of course seal the joint also.
I did a another thing to the riveted belly panels with a louvered vent. I'd refer you to page 1, post 8 of our thread, listed below, to explain my reasoning (its long winded so I won't repeat here).