Originally Posted by roach711
A diode based battery isolater will absorb a little of the alternator output and slightly undercharge the house batteries. A three stage charger is best for deep cycle charging but many RVs use the same setup you're considering. Battery life may not be as long but it will work. You could have the best of both worlds and do both by charging with a three stage charger when shore power is available and using the alternator when you can't plug in. That's how our bus is set up.
The cable gauge needed will depend on the distance between the starting battery and the house bank. There are plenty of DC wire gauge charts on the net. Too big is better than too small. Keeping the cable length as short as possible is also a plus.
I'm going to expand on this, not to disagree with you but to vent a few pet peeves.
What the OP is asking is technically possible, but to do it correctly is not easy and quite expensive.
First peeve ... RV manufacturers don't often do it properly, and they know they are taking shortcuts that hurt the buyer.
To charge deep-cycle batteries from a vehicle alternator is not a smart idea for a number of reasons. The first is that few standard alternators have the required output, so a big upgrade or second alternator will be needed.
Unless that alternator is feeding a DC to DC three-stage charger, it will not undercharge them a bit, it probably won't get them over 80% state of charge. It just doesn't have the voltage to charge them fully, something the DC to DC charger takes care of, for about four hundred bucks.
So RV makers are selling you expensive batteries (if you are lucky), then hooking them up to an inadequate charger that will never deliver full potential, and will wreck them trying. Even the converter/chargers they sell with new units, for charging from shore-power, are often inadequate. They are regulated to 14.4 volts, and that is not enough to fully charge deep-cycle flooded lead/acid.
There are better ways.
You can go down the route of beefing up the alternator and adding a proper charger. Make sure the cables are sufficient.
You can add some solar that will charge while driving, and while not driving.
You can charge via an inverter/charger from shore-power, or a small generator, a very flexible system as the genny can add power for when you need peak-load supplementing.
Trojan T105 RE batteries should comfortably last 8 to 10 years. They are guaranteed for five. Yet all too often we are expected to be happy if RV batteries last five years, or less, while never delivering the power the owners were led to expect.