New owner

6v batteries wired in series will double the voltage, but the amp-hour rating stays the same. The advantage is that golf cart batteries are true deep cycle batteries and usually offer higher amp-hour ratings than hybrid marine batteries, which also will function as a starting battery (which you have no need for). I have two Duracell 6v from Sam's club wired in series which gives me 12v at a little over 200 amp-hours. You REALLY need a battery monitor to keep track of how much power you have, the lights on the wall panel are way too vague and I don't trust them. I got this one, it's cheap and highly rated. https://www.amazon.com/bayite-6-5-100V-Display-Multimeter-Voltmeter/dp/B013PKYILS

While you're on Amazon, the recommendation for the 12v Bible for Boats was a good one. Very easy to read and informative.

Costco is a good place to find 6v golf cart batteries and the prices are very competitive. 

Welcome to Podding!

Congratulations Paul on your 177.  As a 177 owner for 8 + years I expect to get 3 - 4 days per battery when we boondock and when night temps dip into the upper 30's to mid' 40's.  Weather factors like wind and how much we use the furnace during the day will determine if we go 3 or 4 days per battery.  I use the idiot light on the wall to let me know when my battery is at 1/3 or 33% S.O.C., the other way I can tell is when the furnace starts, if it sounds like the motor is running just a tad slower I switch over to the other battery; we carry two as we boondock a lot and don't use solar to refresh the batteries.  What we have are a pair of Interstate group size 24, deep cycle.  We got these when we picked up our 177 back in March 2011, so as you can see, if you maintain the battery, it will last quite a while.  It is a good idea to keep an eye on the electrolyte levels on a regular basis so that there is always water covering the plates.   Also, for the same reason given above, we also carry 2 propane tanks as well.

Order a book from Amazon called "12 volt Bible for Boats".  An rPod is a boat with wheels.  Read it all and you will answer lots of questions you don't know that you don't know.