Refrigeration will be your largest draw off your batteries. It usuallu average anywhere from 3-5 amps per hour depending on the weather, and your insulation. A 70 aph battery really has only 35 amp hours before it is considered discharged. Your two in parralell will give you about 70 amp usable...nort really even enough to run the refrigeration 24 hours.
Fortunately the fridge only draws 300MaH average when cooled down (2-3 amps for an hour or so initially when first turned on) so the two 70AH batteries should do fine for now, and I don't consider the fridge essential equipment, so better to keep it off the main house bank driving the electronics. I minimize lighting consumption by using oil lamps alot, and plan to install LEDs in the nav lights. Heating will not require electricity except for a startup fan to aid draft until warmed up.
My sugggestion is you Keep the AGM as you housebank and your workhorse. Use the other as a second bank only if you runn the main bank down. Echo charger from the main house bank to the starting battery.
I think I mostly agree with your suggestions. Though if I have to have two entirely separate banks, I'd just as soon move the non-essential stuff off the house bank (which at 220total, 110usable is a little on the small side for longer trips) so that there is more available for electronics, nav lights, and other "modern essentials".
With the 60A shorepower charger I have, and two 15A echo chargers for the second bank and starter battery, and an occasional top up from the engine alternator, that should do me for up to a week at a time, I'm thinking, if I closely watch stuff like cabin lights, etc.
I'm also thinking to eventually install a second higher output alternator, though am weighing that cost with adding solar/wind. At the moment, solar is looking like it would be the most beneficial subsequent addition to the system, though a high output alternator may be more of a "sure thing" in these higher latitudes.
For the future it is better to have one large house bank rather than 2 medium size ones you will switch from one to another.
That was my motivation for this thread, to figure out how to build one single bank from the batteries I have.
Also conside 6 volts. Twice as many batteries......same footprint in space.....twice the amp hours usually.
That's my plan eventually, when I have to (and am flush enough) to replace all the batteries in the future.