Steve-I am glad to hear the actual fantastical fan readings. I guess that on low, even with the dropping resistor, it still uses less power than on high.
One ($$) solution is to get a dual marine voltage regulator with dual outputs, running one to the house battery
and one to the truck battery
; then the alternator/regulator isn't fooled by differring voltages of the dissimiliarly discharged batteries and you can get the full alternator output into the more depleted battery
Pete and Rats