I don't have a Lance camper but I do have a lot of experience winter camping down to the -40's C. So let me address a few of your comments on your winter camping.
The first thing is, and probably the hardest thing, is finding a place to fill up your water and a place to dump your tanks. Hopefully your heated shop already has this. Then you can fill before you leave and dump when you get home.
Having a heated shop will mean that you don't have to winterize the camper after every trip which will save you a lot of time and issues.
Myself I wouldn't go through the hassle of adding a diesel heater. When I had my truck camper and my motorhome I just used the propane heater and they both worked fine.
Now that I have a van I do have an Espar gas heater which is similar to the diesel that I use in the winter. I love this heater but running two heaters in your camper would be a little redundant - Space, ducting, carrying extra fuel.
One of the biggest issues with the diesel would be that you would need to have it so that it vents into the underbelly to keep your tanks warm and your pipes warm as well. So you may as well just utilize the propane heater that you would have and just carry an extra propane tank if needed.
If you plan on going to locations that have plugins you could take a electric heater as well to help / cut down on propane use but again remember that having your furnace running will not only keep the inside of the truck camper warm but it also keeps your pipes and water warm as well. You may want to keep the circulating fan on if you are using another heat source.
As far as using the toilet in the winter I do not see an issue with that as long as you have a four season camper.
The thing that I would question would be having a shower in the winter time. I doubt that any place that you would go would have running water and using the water on board in a truck camper to have showers you'll go through your fresh water really fast and you will fill up your gray tanks in no time.
Myself I would suggest in the winter time that you use the facilities at the ski hill (If they have them) to have a shower. It just saves so many potential issues.
As far as having something underneath the bed to allow air circulation that's done as much to prevent mold as it is to keep it warm. Maybe even more so in the preventing of the mold.
My system used to be in the morning after breakfast I would open up the roof vents and the back door and suck in a pile of fresh cold dry air to exchange the warm humid air. I never did have an issue with mold and I still don't have in my current van.
The biggest benefit of a van for me in the winter is that you're always exchanging the air as you're driving. Thew dash air circulates around keeping the van a lot drier than the truck camper ever was. My Class C was the same as the van is in the air from the dash would move around the back and help to dry it out when we were on the road. If it was not crazy cold I would open a rear vent in the Class C while driving to move more air.
Finally remember when winter camping the space/volume of air in the camper is small and you are VERY CLOSE to the walls. I've had a number of people tell me that they can never get warm in a small camper. A couple reasons for this is that when you open the door a lot of the warm air gets pulled out and a lot of cold air comes in so it cools off quickly. Likewise you will notice your furnace will run a lot more then at home as you do not have a lot of air to hold heat.
Finally you're close to the walls when you're in the camper. You're not 4, 5, 6 ft away from the cold walls like you are in the house. A lot of people can feel the cold from the walls and it makes them feel chilled when they're camping.
Not that you cannot do it but just a few things to consider.
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