Just when you think you have fish figured out .............................
Feel your pain - some times fish mess with your head.
From what I hear - the newer fish finders can be tricky to set up. You might want to put boat batteries on charge - then see if it works better.
Guessing -just a guess - the windlass draws amps from a low battery - fish finder " starves " for proper voltage and goes crazy.
If that is not trouble - my dad always had me check / clean the ground wires. 90% of time - this fixes 12 volt troubles.