Exactly what I was thinking.The bigger concern, in this instance, seems to be space rather than cost. Different needs arise to add or remodel than during original construction. I'd also be surprised if it wasn't more cost for the larger wire and charger than for a small inverter and the extension cord likely already being carried.
Putting the dc-dc charger up front needs much larger wire, plus likely oversized to combat the voltage drop since it should be located near the house batteries. Large wire, possibly smaller than with the charger upfront, still needs to be run if space was made to place it in the rear by the house bank. It doesn't sound like there is easy space to run this wire.
Inverters are also cheaper than they once were, especially smaller msw ones. Either case gives multistage charging. Maybe it's less efficient, but it's spare alternator juice not battery storage so it doesn't really matter in my mind. You're not 'wasting' anything. The dc-dc is probably the best option if you're starting from scratch. The second inverter is definitely an option for an after the fact add-on.