Winterizing a finished build

[FAC]

I'm looking for ideas of how you would make a finished bus more useful in wintertime, I'm talking Wisconsin winters for a general idea. Our bus has been "finished" for a few years now, when we built it we really intended to not use it in and hard freezing temps. It does well when the nights get into the upper 20's but if it doesn't come above freezing during the day the cold starts to "seep in"

Anyways it's insulated to about r11 being realistic. Of course the windshield door and doghouse let in the most cold (and heat in the summer time) I'm not sure what to do there, there's just a lot of glass and the dashboard, floor, steps and door basically have zero insulation. Right now the mini split does a great job until about 40 outside temp. Then I switch to the Chinese diesel heater which will keep the back half of the bus warm in any weather we've been in. Which is about 15f outside. Add another diesel heater to the front half of the bus? Enough btu's will make anything "warm" right?

My other questionable area is the drain plumbing underneath. If I spray foam my gray water tank and drain pipes would that be enough to keep everything liquid? Or would I need some type of pipe heater anyways when it gets down in the teens?

[Timeline]

You could set up a temporary partition up front with something like moving heavy blankets for sub zero times.

You could temporarily place a small thermostatic controlled electric heater below your tank and wall it in with plywood/foam insulation.

When it gets crazy cold having electric heaters may save the day. Hope you have that option if needed.

[FAC]

Thank you, I like the idea of having a curtain up for the front area and a backup electric heater makes sense too