Not intending to answer Pauls question but heat tapes are not used ~inside~ the tanks. The are used outside. i.e. I installed an 8' 18 watt tape around my drain station which automatically turns on at 38*. The tape is a/c but runs off of my inverter. At 18 watts it should run for a couple weeks without charging the batteries. Indefinetely if I get a solar panel. My hopes are that this 100* heat will rise through the drains and warm both tanks enough to prevent freezing. The first good freeze we have I plan to drain my gray water and measure the water temp...I'll let you know how it works. My previous coach had cargo heat but lots of LP and $$$ was wasted heating that area when temps were above freezing.
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