My guess is that you are right Hambone...a simpler method would be to use a clamp amp meter and take a reading on the charging wire. I'm thinking a lot more folks have those and for those that don't..they are handy for lots of things and relatively cheap.
In any event...there seems to be a general assumption that a couple hours of charging down the road and you are all set to go. No matter how many amps your battery charger or alternator can put out all the way down the wire to your trailer batteries.... it will take a MINIMUM of 4 hours (and likely a LOT longer) to fully recharge a 1/2 depleted battery. Minimally charging a battery...then depleting it below 50% is the quickest way to early battery death. No matter WHAT the voltmeter says after you've been driving down the road a while... you aren't fully charged if you haven't driven at least 4 hours because you are reading surface charge not the real state of charge.
Here's a decent & wll priced clamp meter for those interested: