This is likely too late, but...
The battery is under the driver's seat. Promasters come with a manual disconnect on the negative cable, so you needn't buy anything or need electric to plug it in.
Pull up the cover, the switch on the negative is pretty obvious. Move the lever to the right and pull it completely off the pole... so that it isn't touching. When spring comes put it back on and the van should start right up. I did this for years with a manual disconnect that I installed on my first Ford rig... also did it with a Toyota Sienna that waited in the garage when I went south for the winter.
Never had any issues with vehicle computers
2019 Roadtrek Simplicity SRT (almost a Zion)
2015 Roadtrek 170
2011 LTV Libero
2004 GWV Classic Supreme