Why don't TC makers use the Truma Combi units?

There is the reason why you don't see them. $500-700 is a few months' payments, enough to make a good many customers choose a different brand that doesn't have the expensive hi-tech water heater/furnace gadget that runs on unicorn farts or whatever...

RV manufacturers know if they go with the more expensive unit, they will lose sales to all the others who didn't.