LOL. Referring to the RV furnace internal design as a "heat exchanger" is a joke. It's just a tin can with a flame inside, not even any fins to help. Efficiency is lucky to be 55%, and the ducting systems supplied in the RV are equally shoddy. It is entirely possible to design a much more efficient furnace without a huge increase in size, but neither the RV appliance makers nor the RV builders want to invest money is a new design, so the furnaces have been unimproved for the last 30+ years. You don't need a larger, high power fan either, so there is no increase in 12v power. Perhaps even a decrease, since a more efficient furnace should end up running less hours/night.
Ditto for RV air conditioners