Thanks @uHu
So for an air heater, using Eberspacher S2 D2L as example, 31W at 12V makes 2.6A amps on full chat, with an 'off' drain of 0.34A until it depletes to 10.5V.
For 2x100Ah Yuasa, in theory this runs for 200/2.6 = 76.9hrs. But that's not the reality. One could halve battery power in...