Here's a photo looking straight up from below my black water tank. It isn't exactly the Ameri-Kart H002 tank
, but it's pretty close. You can kind of see in this photo, and better on Ameri-Kart's drawing, how this style tank has a projection on the long sides. Ameri-Kart has a "holding tank installation information" document which recommends using that projection to carry the tank.
I hacked a crude Z profile late before leaving on a trip and bolted it up through the floor. All four corners of the tank have a duplicate of this bracket. Each Z has a flange that wraps around the corner to prevent the tank sliding out forward or back. I put a piece of threaded rod across the narrow direction and just above the top surface of the tank to tie the brackets together so they can't wiggle apart and let the tank fall through. The nut on the end of one of those rods is visible in the photo. I guess that rod also creates a clamping force on the sides which also helps prevent the tank slipping forward or back. I think I used 16 gauge for those.
This tank is shallow (8 to 10 inches), drains forward, and holds less than 30 gallons so I haven't worried much about the forces of sloshing inside.
As an alternate to using that projection, Ameri-Kart suggests 1-2" wide straps at 12-18" spacing for support of tanks without the projection. I infer "strap" to mean a strip of sheet metal in the 20-16 gauge range -- something the size of the earthquake straps sold for residential tank-style water heaters for example. Judging by that recommendation I'm thinking they're not too worried about sloshing in the holding tank profiles.
Some tanks, especially the fresh water, might be more cube-ish. I haven't entirely figured out what I'll be doing with my two 40 gallon fresh tanks, which are cube-like and nearly twice as tall as my black tank. Ameri-Kart has a separate set of recommendations
for fresh water tanks. Below 30 gallons they say support one side and bottom with 1/2 to 3/4 plywood; over 30 they say support all sides and bottom. I get the idea that sloshing is more of a concern for these more rectangular and larger tank shapes.
As a really rough idea, I'm thinking about threaded rod hanging down from the bus floor with a length of angle iron spanning under the tank. Use several copies of that to get the spacing right. Insertion of plywood on the sides and bottom is easy. Ends not so easy. But plywood isn't preferable because of the weather-exposed location under the bus, so maybe use sheet metal instead? Whatever the sheet is, it can be secured to the framework to stiffen it and prevent sliding forward/back from sloshing. The right kind and attachment of the sheet can fill the role of the threaded rod, too. Come on, it's late and I'm not thinking very well.. somebody step up and improve the idea!
Regarding the 1.5"x1/8" angle -- if it's arranged such that it spans the narrow direction of the tank, and is repeated at 12-18" spacing, then it's plenty thick. If you box the tank like Ameri-Kart suggests then simple sheet metal or flat bar straps could do.