Tell them to show you the FAR.

I don't think the EPA has such a rule.
In the 1996 phaseout of leaded fuel, EPA exempted certain activities. Besides aviation, vehicles for off-road purposes like racing cars, farm equipment, and marine engines can use leaded fuel. Several gas stations where I live still sell leaded fuel for "off-road" use only. Some of the brands are VP, Sunoco, Torco, etc. I believe NASCAR switched to unleaded fuel in 2008.

Just get an old cherokee gas tank and put it in the back of your truck. Not much they could say then cause it's going directly into an airplane gas tank.