I can't put on more clothes which will make coffee. Before I installed that propane heater I used to heat a 2" industrial nut with a rosebud torch and set it atop a hibachi pot on the passenger side floor. Crude, expensive, need lots of tools, but 10 minutes of soaking inside the cab the temp gained eighty degrees F.
With regards to ANY electric water heating coil system. With hard water the coil crusts up. The manufacturer tells you to clean it with vinegar. Pure ignorance of chemistry and reality.
Clean with vinegar
THEN
Follow with a baking soda solution flush
The dissolved minerals come out with the baking soda flush. NOT with vinegar alone.
Same with flushing a car radiator or rigs hot water heater. They are a hassle to flush plumbing wise but ONE GALLON of white vinegar then a fill with water, an overnight at the highest dial setting will get the tank ready for a baking soda flush. Four boxes dissolved. Another night at max temp then lookout -- here comes the mud.