I would say don't use your bellybins at all! I suggest hanging your house batteries, propane, water and waste tanks, and anything else like a generator, directly from the frame rails and bodyside. Yes, it's a lot more work to do this, but the advantages are two-fold: A) Obviously you keep all your underfloor storage bays for storage (and yes, you'll be very thankful to have that space), and B) if/when anything leaks it will drip harmlessly down onto the road, not create a festering swamp (or worse) inside your enclose bellybin. Imagine your propane cylinders leaking slightly - where would you rather have that propane go, into an enclosed space or harmlessly dissipated away into the air flowing under the bus? What happens if a battery case splits - do you really want sulphuric acid swilling around inside a storage bay?
To give you some idea of what's possible, my bus has an 8-feet long full-width 'pass-through" underfloor storage bay, but I've managed to also fit under the floor all the following:
1) Spare tire/wheel (with its own winch and wheeled dolly) under the stairs;
2) Generator between the door and front wheel, and a quick-connect outlet for regulated propane (I converted the generator to run on propane);
3) A vented storage compartment for anything that could leak or smell (I keep spare oil and fluids there), ahead of the left front wheel;
4) A 120 VAC air compressor above the front axle between the frame rails, tied into the bus's accessories air system and with three air outlets around the bus for running tools or inflating tires;
5) Two 110-gallon water tanks either side of my fuel tank, hanging from the frame rails and the floor's longitudinal structural supports;
6) A quick-connect outlet for unregulated propane (for a BBQ grill), and some access to the fuel tank's filler neck and pipe (just in case it ever leaks and needs attention) and to the two water tanks' outlet valves and drain valve;
7) A water compartment that has two pumps on a slide-out tray, an 8-branch distribution manifold, whole-house filter, 2-gallon pressure accumulator tank, water pressure gauges and valves, all in a space less that a foot wide next to the left water tank;
Two pull-out trays for eight golfcart batteries, open to the ground and with all the house batteries primary switchgear, with the propane distribution manifold above them, and with a 1.5" dump valve for curb-side grey water emptying, just ahead of the right rear wheels;
9) A 115-gallon grey water tank under the frame rails, just ahead of the differential;
10) A 65-gallon poo tank and the three main dump valves on the left side just ahead of the rear wheels;
11) A vented compartment for two 20 lb propane cylinders on a pull-out tray and their regulator and master shut-off valve, fully compliant with NFPA 1192;
12) A pull-out tray for the two Group 31 start batteries, just behind the right rear wheels;
13) A hydraulic fluid cooler for the radiator fan motor and a remote transmission fluid cooler with fan, between the left rear wheels and radiator.
All this is in otherwise empty spaces under the floor, and nothing is using up any of my precious bellybin space that I can keep for Other Stuff. There's still some space left under the driver's seat where I maybe could put a mini-split A/C condenser, but that would require some serious finaigling to make it fit there. My intention is to not have any unused space at all under the floor - I think I'm getting there!
What's up with that stupid yellow blob thing that shows when I type Eight) ? Is my computer possessed, or is it a forum glitch?