Yea if it's not being hauled too often I wouldn't worry about using a diesel tank, provided it is safe to transport gas in a common sense way, not a government safe way. Even in the worst case of an accident, what are the chances it punctures the tank, the tank happens to be full, and they actually care/check what is in the tank. I wouldn't run a placard and if ever stopped say it's diesel.
Just because you were hauling gas illegally and got in an accident wouldn't mean your insurance is void. It would have to contribute to damages in some way to be denied. Like if someone hit you and truck caught fire, they might try claim it wouldn't have happened if it was diesel, but still good luck with that. Assuming two gas vehicles are in an accident and everything burns quite a bit, insurance would have virtually no way to ever tell what was in the slip tank. They would view the vehicle a few days later burnt in a yard, and likely not ever care what was in the tank.
Just because you were hauling gas illegally and got in an accident wouldn't mean your insurance is void. It would have to contribute to damages in some way to be denied. Like if someone hit you and truck caught fire, they might try claim it wouldn't have happened if it was diesel, but still good luck with that. Assuming two gas vehicles are in an accident and everything burns quite a bit, insurance would have virtually no way to ever tell what was in the slip tank. They would view the vehicle a few days later burnt in a yard, and likely not ever care what was in the tank.