A hydrometer is in my opinion the best way to check state of charge on a battery. Take a reading when your batteries are low before you drive to the next spot, and then take another once you arrive. This will tell you how much of a charge you really got during the time you towed.
I suspect you are probably only getting to the neighborhood of 70% which is a bulk charge. I would also guess the voltage drop in the charge line is too great to get a productive absorbtion charge. Absorbtion is the stage that gets you to about 95% capacity.
But as I stated, a hydrometer check is really the only tool you can bring on the road that will tell you where your batteries are really at. Google handy bobs solar charging puzzle for more info than you ever wanted about RV battery charging.
