Shore power
 

How many people plug the boat in while not using it? We were thinking of keeping our 36 plugged in to keep the batteries charged, but we have never done this with prior boats. Is it really necessary? Are there any downsides to leaving the boat plugged in?

On the trailer or in a slip?

We keep the cruiser plugged in 24/7... 365/yr, as with every other cruiser in the water. It's an in water slip kept boat, so gotta keep the fridge running and batteries always charged so the bilge pumps will work if the occasion ever arises. Plus the Mercathode system is always drawing on the batterys. You don't want to loose that protection with dead batteries.
The Fountain was lift kept and it was only plugged in when we were there to run the A/C and the outlet, however the battery charger was only switched on when we were jamming to tunes on the dock.
As long as your charger drops to a float/maintanance stage to keep the batteries topped off and not boil them, you'll be fine. Still need to check the water in them though.

The boat is kept on a lift. I'm fairly certian that the battery charger is a trickle charge so it shouldn't fry the batteries.

I keep mine plugged in. Seems like I'm always leaving something on. And I don't think my batteries are in top condition anymore.

shorepower
 

I keep ours plugged into shorepower while on the trailer in the garage. The battery charger is active. A seasoned RV user taught me it is best to keep the refer on all the time also.

Here is a hint-If your AC line is on a GFI circuit, turn off your Mercathode system before you connect the shorepower cord to the AC line. The Mercathode system will trip the GFI circuit. I do not know why this happens, but it does.

If we leave the boat plugged in, we need to leave the battery switches on as well so the battery chargers work, correct? Is there any way to tell that the battery charger is on?

If the battery charger is wired directly to the batteries I dont think you need to have the switches on, at least the way mine is wired you dont.