Combining old battery bank with new battery bank

If you really think the old are down 50% capacity I would trade those in for new.
Then you have close to 900 a/h :)

You are really not gaining much to try to keep the old stuff working.
JMHO

DISCHARGE AND RECHARGE rates will be different
use a selector switch between the TWO banks
and you can use the new bank as the main bank and the old bank as backup
be sure to use them AND charge them separately

I wouldn't recommend mixing old with new but if you really want to carry that many batteries, use a Perko switch and keep the battery banks separate although you won't be able to capitalize on Puerket effect. Plenty of stored 12 volt power is important when camping off the grid, but just as important is the method used to replenish battery bank. We use our genny for high current draw items for an hour or so a day anyway and our Iota IQ4 converter does a great job of quickly replacing consumed amphours. Converting to LED lighting a couple of seasons ago nearly had the effect of doubling our 12 volt reserves while off the grid and is an essential mod if most of your camping is done without shore power.

Add one of these and use the batteries as two separate circuits.

Battery Selector Switch

from what I know about batteries your not suppose to mix old and new.