I guess it seems reasonable. Some things to possibly be concerned about
- how's the condition of the coating inside the steel drum? Water will condense inside the drum and eventually create a rust problem.
- which orientation? Laid on its side and mounted under the floor, I assume, and perhaps longitudinally (ie long axis parallel to the length of the bus)? I'm not sure whether sloshing might be a problem; internal baffles to limit slosh might be important. Obviously the sloshing of a 55 gal drum isn't going to be a vehicle stability problem like it is with a tanker trailer, but in a container not designed to withstand such forces, might it cause container failure?
My skoolie drinks diesel but I've been thinking of adding an automotive gasoline tank. It'd be handy for fueling a gas generator, and it's a convenient way to dodge the fire regulations about home gasoline storage (not all my vehicles are oil-burners!). 'Round here two 5 gal containers is officially the limit, but fuel tanks in vehicles aren't subject to those rules.
Some older SUVs (I'm thinking Grand Cherokee) and probably some pickups too have a nice relatively rectangle tank that might be a good candidate in the 20 to 30 gallon range. The filler neck from the tank donor could be useful too.
My '91 Blue Bird had two tanks: round cylinders, about 30 gal each I think, and plumbed together with something around 1/2" tube so that they'd draw down evenly. This made filling the tanks obnoxious because I had to fill one, then fill the second (meanwhile some fuel from the first is flowing back into the not-yet-full tank), then return to top off the first again. If you had separate fillers and an isolation valve you could prevent this.
I had one mishap in which I parked the bus on a slope after filling both tanks. This put the fuel level in one tank above the filler cap, or overflow/vent, or something else on the other tank.. a fair amount of diesel bled onto the asphalt. I learned that spilt diesel softens asphalt and that spot on my driveway has never been the same..