[VAL] going to two batteries

[Chris Koehn <timberguides@xxxxxxxxx>]

Hello all-
I'm interested in replacing the standard lead acid battery in our 1979 
@ 31' with (preferably) two glass mat batteries to greatly increase our 
ability to go without A/C power for longer periods of time. Our coach 
came with one lead acid, accessed through it's own door from the 
outside front. The compartment is sealed from the interior of the coach 
and can accommodate only one battery.
What's my best course of action re: installation of two batteries-  in 
parallel? My fuse panel has "battery #2" AGU 50 fuses installed (there 
are four AGU 50 fuses total: one positive for battery 1, one positive 
for battery 2; one negative for battery 1 and one negative for battery 
2). I'm not sure if this would have been for an optional second 
battery, and if so, should I consider wiring the batteries in to the 
fuse box separately?  Lastly, do glass mats need to be vented, and are 
they still considered "state of the art"?
I use a 60 amp  Intelli-Power and charge wizard.

As usual, thanks in advance for the sage advice!

Chris Koehn
currently in SW New Mexico