I found a nice little heater that I run from my van that gives me 140 degrees water, it is capable of 80 - 165 degrees, I run water from my water tank to transfer pump, transfer pump to heater, heater to M5+ A/F, it called Eccotemp L10, folks use it for cabins and such for hot water, so far it has worked great, it keeps my M5+ with hot water constantly, it shuts on and off just like a transfer pump, its worth looking into if you a tight on money and would like to have hot water....
CHEAP porty heat
CHEAP porty heat