Naio,
I noticed a couple of things in your last few updates.
You said the van's starter battery was sitting at 12.68v. In all likelihood, the battery was full (or at least very near full). So it was good to go. And besides, you're running the loads off the agm's, so as long as the starter battery is being kept charged, all is good, in that regard. So there likely was no need to charge it up.
When you were charging the starter battery, voltage from the alternator was at 14.5v... this is a good thing, and with a small load, and the peukert effect, voltage was 13.87... so all good. But then you said you checked on things, and "Chassis battery back up over 14". ??? Was this at rest (shortly after being charged), or while being charged? If at rest, that is surface charge, fine. But if while still charging, voltage had dropped significantly (from 14.5) then that's not a good thing.
As others have stated, it may be better to go directly from the alternator to the agm's. All that switching, from dc to ac, back to dc, is causing a lot of waste/heat. And I would have those two 50a agm's in parallel, if you don't already, both for charging and discharging. The 100a I would leave on its own, and use one 100Ah bank while charging the other. At night you could use both banks... one for the fridge, the other for heat. Use the larger inverter for the fridge (provided it works ok wih the msw), and a small inverter for the heat pads, etc.
As for a clamp meter, Home Depot has a wide selection, but you'll get a better price on eBay. Seeing as you're now moving to a site with hook-ups, it can wait. Order one on-line now, and you'll have it before your next outing. Plus you can test things at home, before leaving, to see what works best. nudge nudge wink wink ;)
I have this oneHopefully you can get at least 25a from the alternator to the batts, for a decent C/4 charge rate on a 100Ah bank. If that's the case, you'll be able to do a 50-80% in just over an hour, and a 50-90% in about 2 hrs. At 14.5v, you'll be at about 90% soc when amps taper to about 5a.