Is it wise to keep MH plug in to Electric?

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.

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.

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

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.

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

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.

in the months i use the rig all the time, i leave it plugged in and the refer on with stuff in it. in the winter, i unplug the rig xcept for about 1 week a month just to keep the batteries up.

In the last 5 years since we've owned our rig, I've had it plugged in for the majority of the time....either sitting in the driveway or at a campsite. When I store for the winter in December, I turn off the main battery switches and the rig sits under cover at a storage lot until I de-winterize just before Easter. I've had no problems as a result of keeping her plugged in.

I keep our 96 Southwind plugged in 24/7 when parked in our yard.

I've been RV'ing since the 60's. I have always kept my rv's plugged in and the frig running. I winterize during the winter and the rv is ready to go camping on a moments notice. I have never had a frig go out due to it running 24/7/365, never. I just made sure the converter doesn't boil out the batteries in the old days, not the converter shuts off when the batteries are fully charged