First step is to cut a gaping hole in the side skirting of the bus. This is the rear on the driver's side. We chose this location because the propane appliances are on this side of the bus, and this is the largest (and most easily accessed) available underbody space.

Next we fabricated a box from materials leftover from previous projects:
3/4" plywood bottom
5/8" plywood sides
5/8" OSB top
3/4" pine back panel
1" angle iron frame

The box interior dimensions are 30" wide, 20" high, and 28" deep.

Primed then painted (inside & out) with the gloss almond latex we used for the interior.

1/2" all-thread (threaded rod) holds the box snug to the bottom of the bus body. We'll trim the excess all-thread below the box at a later time.

The box sits a bit forward in the hole due to the presence of a bumper support at the rear (just visible near the upper right of the box). Moving the box forward a few inches allowed us to maximize the depth of the box while avoiding building an odd-shaped structure.

1/2" hardware was also used to fasten the tank bracket to the bottom of the box.

We intentionally left a gap and the bottom and top of the rear of the box for ventilation.

The tanks in their new home.

For a door we re-used the bus skin that had been cut away. First we cut a stainless steel piano hinge to length and screwed one leg to the bus skin.

The hinge was then screwed to the bus body.

The tank hangs about 8" below the bottom of the bus body, so we used (again, leftover) 26 GA sheet metal to cover it.

A door holder has been added.