Battery Charger

You have shore power available so why not use the on-board converter to keep batteries up?
You could jumper to the chassis batteries if it is not equipped to do that.
Make sure you keep an eye on the water levels!
A newer converter would have benefits.
A charger such as this would work. Battery charger

I have removed my batteries for the winter to my garage and am looking at 6/12 volt charger to maintain them. I'm thinking of a .75 amp to rotate every couple weeks to each battery.

+1 to just plug in the main cord.

I second what enblethen suggested. Not only is it a battery charger, it's a maintainer too. When the battery reaches 14.4 volts the charger will shut down and come back on line at about 12.8 volts Good stuff for the price.

Richard

OP your rig is a 1989 . I would not be removing batteries due to the fact the converter may not be a smart one.
OP, you need to verify voltage output of your converter. Please post with a battery and without. Make and model of converter?
If you decide to use battery charger for chassis, I would not use any but a smart charger.

If your converter is not up to modern Smart standards I'd upgrade by replacin the "Guts" (electroncs) with a Progressive Dynamics 4600. or if it is a stand alone 9200 (the difference is mostly packaging)>. I just had to replace my 9180 (I have the charge wizard)

No better way to keep 'em charged

For back up (I had to swich to back up) a Battery Tender. the model I have is 1-2 amps "Tender" 20 amps "Boost" (Must be manually initatied) and 70 anps Jump Start (likewise) it took 24 hours in boost to re-charge my system when the old 9180 died 9 days ago