I would suggest installing a continuous duty solenoid (NAPA has these, ST85 is the part number I believe.) between the positive points of the chassis and house batteries. Preferably not the actual posts as this is a corrosion point and the fewer the wires the better. Use a junction block it at all possible feeding it with a large gauge battery cable. If you wire the pull in coil into the accessory side of the ignition switch it will keep from connecting while cranking but will allow you to charge both sets off the alternator while driving. The big risk is if you have run the house batteries way down and then parallel them to the chassis you run the risk of over charging the chassis batteries while driving since the alternator is going to pick up the low voltage on the house batteries so the amps will be cranked up to charge them. Alternators are a battery maintainer not a battery charger.
Option two is to put the continuous solenoid on a switch so you can control when the batteries are paralleled.
I do not recommend battery isolators as what they actually do is click back and forth between the two sets of batteries and have a tendency to cause alternator failures due to the no mans land when switching so the alternator goes gang busters charging nothing and shortens their life substantially
Lastly if you were to install a good quality on board battery charger for the chassis batteries I would suggest at least 5 amps per battery (Group 31 for most trucks) plus any other loads you may have like flashlights or tool battery chargers that are not 110 volt. If you wire it into your shoreline connection it will always keep your chassis batteries up too.
Good Luck!!
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