Inverter charging the camper from the trucks batteries.

Wouldn't the efficiency loss be realized only in fuel economy which should be inconsequential?

I'd agree with the first part -- the engine is the energy source in both charge methods. Don't know about inconsequential - but probably. You - as in not me - have to know all the factors and do the calculation.
Heavy duty 12v is cheap, easy, extremely well tested, and known to work.