I am lately having problems with the letter U coming up on the screen and the pump stops but when I turn the power off and on again it seems fine but is very annoying when your a hundred meters from the van,is it on the way out and is only a year old!
U is the unit cutting out at 10.3v to stop you draining the battery totally flat and damaging it.
If your battery is good, then the issue is probably a poor connection somewhere along the line. A poor connection creates resistance and restricts the flow of current which the controller is assuming/interpreting as a flat battery. First suspect is the battery terminal and then the inline fuse holder.
Do add an inline on and off switch 10 amp to isolate the varistream at night between your battery + and the Varistream Battery +. They consume current even when switched off, so you battery could be flatter than you realise.