The average system doesn't have three tanks. It usually has two, with a small diesel tank and a very large WVO tank.

Resturant supply stores are usually open during business hours on weekdays. Fuel stations and truck stops are open 24/7.

Okay. so scratch the lend truck situation.

There are many situations that can occur that would require filling the heated tank with diesel...

Fuel leak in the diesel system but not the WVO system...
Problem in the switchover valve...

I wouldn't like to be limited to a 15 gallon diesel tank when I have a 40 gallon WVO tank.

I'd just rather not destroy a lift pump or set the fuel in fire or something. But I guess if the temp is regulated at only 160 deg Farenheit, it should be safe.