you can either go with lots of solar and try to get to 100% every day, or look at how much you can "loose" every day such that at the end of the trip your down to 50 or even 40% of capacity.
For example, I have 4 T-125's. with 120W of solar in the fall and only getting about 5 good hours of sun, we went 8 days before the batteries were at 50% and then the trip was over. So basically every day the batteries were going down by 6%SOC. Then when I get home hit them with a good charge to get them back topped off.
Now if I would have had my 100' of cord instead of only 50' I could have positioned the panels to get close to 7 hours of good sun. An extra 20AH/day would have gone a long way!
