Adding a 2nd 'Similar' but not Identical Battery?

The battery you are looking at is a 65 amp hr battery, as a pair 130 total. Of which, as marine/rv batteries you can only use about 65 amps without over using them.
If you change to GC-2 batteries you would be able to use up to 75% of the 220 amps available.
I don't know what gc batteries will cost you up there but the difference in cost, and that you have to buy at least one no matter which way you go makes the upgrade to 6 volt batteries seem sensible.