Y'all add on to it.
if you haven't been on the water in awhile. Check the water in your batteries. Use distilled water.
When you hook up your onboard charger, make sure your lights are saying it's taking a charge. I have a MK 330 onboard and I hooked it up and looked the next day, 2 of the lights were flashing green while the other was solid red. The flashing green means it has a full charge while the red means you have a problem. On MK chargrers, the lights should be an Amber color when it's taking a charge.
The battery had lost to much charge to take a charge do I hooked it up to the portable charger for an hour, then replugged it in. It's taking a charge now.

Oh, check your water in take and pee hole for dirt dinner nests. Mine were. Lol