I'm not an advocate of running the pump off the van's starter battery.
We tried it with 3 different vans, and the starter battery was flat in 4 days. We later replaced van number 3's battery and tried it again. Same result.
I need to recharge my van's starter battery every couple of weeks. Our neighbour down the road has the same van except 2 years younger. He's a plumber, and he needs to do the same.
Temperatures have dropped a bit, might be your controller needs calibrating , pump tricked into thinking it's on a low flow. To check if this is the issue, disconnect half inch hose from hose reel and just run/return water back to tank directly from half inch hose, leave running for a few minutes going through all the flow rates/settings then return it back to where you want the flow, connect back to hose reel and see if pump is running correct speed.
He could just try and run the engine when the pump slows down and see if that rectifies the situation. I concur with others that it sounds like a battery state of charge issue, either flat or needs replacing. The other thought could be a faulty connection. A faulty connection can heat up with resistance and restrict current flow. The fuse holder is a prime suspect.
The other question is; is the tank venting?