Can I charge the house battery with the engine alternator?

sounds like the wiring is already there, add a battery isolator at the engine side of that battery cable coming from the house and wire it up to the starter battery.

Yes a DCDC would charge quicker, but a simple isolator is like $50 vs $150+ for a DCDC. And since your RV is older, it probably won't have a smart alternator so it will charge quicker than the newer RV's do.