Good question. Do you have a dual battery set up, or just a single battery? It could be a sign the battery is going bad, but if you got all that drain for hours and hours on end, it will happen. Heck, I have issues with everything drawing on my house battery in my dual battery set up and I am considering a solar powered trickle charger myself. But, that said, I’ve gotten by without one these past 8 years, with only a couple hiccups along the way (mainly due to Sea Legs battery draw/consumption).
Recently we simply started bringing our wireless BT speaker with us on the boat to handle those down times anchored up or on the sand bar. Then I can simply turn off the entire boat electrical system to save the drain on it.
Now that said, I am assuming you are seeing this issue due to running your electrical system for music, chart plotter, etc… while the motor is off. If you are noticing this problem WHILE running your motor, then its certainly a sign something is wrong…and easily most likley the batteries going bad.
How old are the batteries?