This quoit is from Yandina.com a expert in the field of battery charging.
"Although the conventional recommendations are not to mix old and new, good and bad, BIG and SMALL, etc. in parallel, those misconceptions actually only relate to batteries in SERIES. You should understand the reasons and make up your own mind. The truth is that you can parallel just about anything. The downside is, if they are not matched, one battery (the newer) will tend to carry more of the load and the total capacity will be slightly less than the sum of the individual capacities. If your new battery has a capacity of 100AH and the old one still had 45AH left, then in parallel you will get something around 145AH, where had they both been new 200AH would fit in the space, but it will help and it will work.
When the old one eventually dies, it will drag the new one down but this happens whenever two batteries are in parallel and one dies, the only difference here is it is going to happen sooner than it would had they both been brand new. So big deal, you gained some residual use out of the old one and the new one will not be permanently damaged - just charge it up again.
The bottom line is the batteries in parallel provide more power - never less - than either one on its own - it just may not be the "ideal" way to do it."
This is my understanding,
Every time you plug in your charger or start your engine, you are creating a large bank of different size batteries.
As soon as that clunk of the Big Boy or BIRD solenoid is heard, large cables connect the positive cables of house and chassis batteries.
They would only separate during long duration, high amp draws, pulling the voltage of both banks down below 13.2. Than the affected bank is on its own. ( heavy inverter use or engine starting ). Once the affected bank recharges above 13.2, it reconnect to the other bank, creating one big single bank of batteries again.
While boondocking the banks disconnect, to protect 1 bank from discharging below a level needed to start the engine.
I bought my old MH 16 months ago with a brand new 12 volt house battery under the step. I added a pair of 6 volt ( in series ) in a compartment nearby and connected them to the 12 volt. It tripled my capacity for my many Wal-Mart and Flying J nights, and has not caused any problems with any batteries.
Just my ramblings.