I would not bother with trying to connect the chassis batt to the house batt for this.
I assume this is a motorhome?
There should be an over-ride button so you can self-jump start by connecting the house battery to chassis battery when starting so the starter motor has power from both, but maybe not.
I assume you are storing the motorhome for long periods is why you need to charge the chassis batt?
The easiest way is to put a 5 or 10 watt solar charger on the dash and plug into a DC outlet. You need the outlet that is connected to the battery (has a battery icon) not the one connected to the ignition (has no power when key is off).
If the windshield faces the sun at anytime then this will work.
i had one on a E350, I think it was only 5 watts, and it did the job. It actually put in all 18 volts, but never was a problem for 10 years because 5 watts is so little, but one that puts out 13.x is preferred.
or, as others have said, if you are plugged in anyway, then just put a small trickle charger on it, you can plug into the DC outlet also, so no need to pop the hood.