I've never driven a septic-liner, but I used to haul fuel and propane in tankers. (Not simultaneously, nor with the same tanker.)
The fuel trucks were double-walled, but not on all sides of the tanks: the tops were single. (If I remember correctly)
The propane tanks were single walled, but had heavy production and testing regs. Type and hardness of steel, thickness of walls, type and number and location of pop-off valves, bleeders, belly-valves, fill valves, and vapor vents were all established by law and subject to inspection.
The requirement that sticks best in my mind is the requirement that all fittings on the tank had to be armored against knock-off and roll-over. This resulted in large collars and cages around fitting that could trap your hand in some emergencies.
Highway tanks are no joke, guys who work around them are very careful, depending on what they haul, or else they are injured. There are few careless and uninjured tanker-drivers around. I have an otherwise exemplary safety record, but I did manage to shoot myself in the thigh with a jet of liquid propane, causing multi-thickness chemical burns that laid me up for a week, and put me in shorts from Thanksgiving to Christmas in '99.
John has a point, especially when he mentions cold weather, but I wonder if a properly prepared enclosed trailer or box truck could comply?
The fuel company I pulled for used to set single-walled fuel tanks in steel containment tubs in order to comply with regulations. They function like bilges, and could possibly do so on a trailer or truck as well.