Is it wise to keep MH plug in to Electric?

My old 5th wheel would over charge the batteries so I added a disconnect and a simple battery charger that was a maintainer and would float when the batteries were charged. I got the charger from Costco. The disconnect I picked up from an RV place it might have been Camping World. It was a simple bracket that had a plastic insulator that isolated the positive terminal from the trailer it was easily installed and removed.

With this setup I left the 5th wheel plugged in. I ran a heater on a thermostat controlled plug in freezing temps and it gave me piece of mind even though I winterized it. I also like to leave my frig on.

That would be my concern if the unit sits for a long period of time

Probably not, with the chargers on older Winnies you should alternate every couple of weeks. You batteries could be over charged. I can't explain the workings, but I know from experience.

It should not be a problem but I wonder why you would.

We take ours out of storage about April 1 and put it back in storage about December 1. Between those dates, any time it's not traveling it is plugged in, refrigerator stocked, ready to turn the key and go.

I keep mine plugged in to run the ac on low for humidity control.
It’s Florida, kept outside (mostly shady).

If it was under covered storage, I wouldn’t leave it plugged in.
I would crack roof vents and run a few fans.