Should I leave the MH plug in all winter?

There is another option if you do not have a "smart" converter. You can plug in, disconnect the batteries and hook a battery charger/maintainer like a Battery Tender/Battery Minder/Noco Genius to them to keep them happy. I have a 10 amp Noco genius mounted in my battery bay even though I have a PD converter. I use it because if charging/maintaining off the converter and AC power fails, the batteries will be discharged from parasitic loads. Not so with the separate maintainer and battery switch off.