Gel batteries question?

New set of Gel’s last year, 6 Deka 8g31 Dominators, for my 2005 E2. Work great until down between 3 & 4 bars, probably around 65% or so. Then I lose power and speed gradually, eventually not even making it up a small incline and slower & slower speed. This is usually around 12-14 miles, so no problem since I don’t take it too far from home, but can only use for a few miles beyond 3 bars gone. I just plan on putting on charger then and don’t try to push it. Any help out there for what I may have going on?
I have been advised to run the batteries down to about 20% or so once in a while to help extend the overall life, but of course, haven’t been able to do that…HELP??