Three 12 volts all new same brand batteries ( one under the pasenger sea,
two others installewd under the rear seat ) are now in my Westy. Each is new
and has 65 AH. Under the rear seat posts of both  batteries were connected
in parallel : positive posts interconnected and 4 AWG common cable attached
to starter terminal. Now  positive posts of all 3 batteries are connected to
the starter. Very simple setup, no switches , relays, etc.
I had before only one aux. battery under the rear seat. For 4 years it was
connected to the alternator with 8 AWG cable with 30 Amp. fuse in line. That
fuse once blown, I understood that the battery helped starting and this is
why the fuse had blown.
With 195 AH on board I am secure. I obviously must watch not to drain the
batteries too deep. And that is easy. I never kept batteries for more than 3
years in New Jersey, USA.  Portugal is warm and batteries shall last longer.
Ark