I would not hesitate to occasionally boost house batteries via large cables from my truck alternator. As far as alternator speed and output, my F250 155a alternator is capable of providing about 90 amps at 650 engine rpm (2000 rpm alternator). I hacked into the PTO harness and have a high idle switch set at 1450 rpm, which would provide about 150a of charging amps at 80 deg. I didn't measure the current, but got my numbers from Ford's alternator specs for the 6.2l 155a alternator output curve.
To get my modest 230ah bank from 50% to 80% at idle is going to require running the truck for about an hour, 30 min at high idle. That last 20% to 100% is not feasible. These are ballpark numbers - but IMHO show it's a viable option to eek out a day or two of boondocking. Really a better method would be to use a small 1000w generator and either the trailers converter or a 40a automotive battery charger.
