Trolling battery question

I have 12/24 set up. Trolling motor 19 yrs old. I have three bank onboard charger that is plugged in unless fishing. Batteries are two years old this Tuesday. Go to lake run on calm day all day rigging, lite use bumping to move slowly, all well. Windy day, running on high 12 or lower 24 after a few hours on the lake less and less power. Tie up or drag chain let it set awhile and more power back, not seemingly as much as at start. Come home check with volt meter, 12.11 and 12.8 in the other. Put meter on battery and run trolling motor voltage drops to 7-8 volts. Let off and voltage back up. Plug charger in, leave overnight, shows full charge for all. Voltage is 12.87 run trolling motor, voltage drops to 12.5 or so. About the same on 24v side drops to 23.9 while running. Took one of the batteries out and checked fluid all full, checked with Hydrometer and all cells are in the fair range. I do not know how accurate they are, never used one before. Is there a true way to load test a deep cycle battery?

While having the problem my thought was the trolling motor was drawing to much current, getting hot, let it cool and works again.

Now thinking the batteries are just getting weak. I just thought that the batteries would show a lower voltage on a standby reading. Did not get to check voltage while having the problem, probably 3 hours after having problem voltage reading was taken. Would the voltage come back up just sitting if they were down 3 hours before? But lack the amps to run the TM!
Thanks for reading, and any thoughts are appreciated! Mark

I could have sworn there were half a dozen answers to this question this morning.

That was on the Mo board, I posted it there also! Thanks

Thanks, I thought I was losing it.

I'd forget running on 12V. If you draw the one down you don't gain anything going to 24 because you have one drained battery. 12.11 is deeply discharged and 12.8 isn't discharged at all.

If the TM can actually drop the battery to 7 or 8V, the battery is shot. My guess is you are measuring on a cable rather than straight at the terminal, and you are seeing cable or connection loss which may be your underlying problem. Make sure everything is wrench tight and see what happens.

Batteries tend to rejuvenate after sitting as the electrolyte mixes.

I fully agree with what catman had to offer. Just for what its worth, I change out my batteries at 24 months (period). They are getting tired.!!!!!!! You probably have noticed no real probs on those calm days and half what you expect and need on the windy days. I know its a hastle and everyone would like to eek out that last month. Face it, its a new season and believe me you don't want the hastle. Don't ruin a good trip in the future because you took a gamble. They only have so many recharges.