Hi Robert
Welcome to Sunline Owners Club! Glad you found us.
There are a few posts we have of folks who have upgraded some items on the standard Sunline to help in cold weather camping.
Here are some for further reading.
http://www.sunlineclub.com/forums/f69/what-the-coldest-temp-that-you-have-camped-in-10814.html
http://www.sunlineclub.com/forums/f65/winter-upgrades-finally-completed-9882.html
http://www.sunlineclub.com/forums/f65/adding-some-insulation-9937.html
There are a few more but I can't find them just yet. I'll hunt some more as there is one on what a guy did to his that was amazing. I have not seen him post in a few years but his post is still here.
A heads up on the condensation, you have to deal with it or the camper will turn into a sweat box. Once you are in the camper and the heat is on and it is really cold out, that is when the problem shows up. Windows and walls will get soaked. There are 2 ways out of this. A dehumidifier or to vent the camper. We do the vent trick. I open a roof vent a good 1" opening which is more then 1" of lift. Especially at night. The moisture from us sleeping is unbelievable. When you cook, crack the window open. When you shower, open the vent in the bath room a good inch. You can close those back up once done, just leave the one roof vent open. We pick the one farthest from the bed room so it does not freeze us out. The moisture is dying to get out so let it go....
Yes you will loose some heat that way too however venting is the easiest way to do this.
We also do the plastic shrink wrap on the window. We just finished this up for this winters camping. If you are doing a quantity of winter camping this really helps. Keeps the heat in better and really cuts down on the window sweat. There is a post of mine on here if your interested I'll dig it up.
You mentioned holding tanks, tell us what day time temps and what night time temps are in your area? If the ground is not frozen and the day time temps get up in the 40's or higher you can get away with more. If the ground is frozen solid and the days and night do not get above freezing then you will need tank heaters, enclosed tanks or the anti freeze trick at least for the black and gray. The fresh tank, is yours inside the camper or under it? That one you need to deal with if it is out side. The tank heaters are an option for this one.
Need more info on the temps in your are on things to watch out for. Even the outside shower can be a problem pending how cold it is.
You mentioned to make sure the truck is running if you want to run the furnace, well not really. It will not hurt but is not a major issue not to run the truck, especially at the price of gas now a days. The furnace will run off the camper battery, just make sure it is fully charged before the trip. Now the furnace will drain down the battery as the fan sucks more 12volts then anything in the camper. Running the furnace to heat the camper up may take 15 minutes. That is OK on a good battery. Doing it for 8 to 12 hours trying to get the camper to stay at 70F in 10 deg F weather with the wind blowing may be a problem depending on the battery size. When hooked to shore line power, we use the furnace as a back up and use a small ceramic heater and or an oil filled one. On your little camper, 1 ceramic heater will do.
Hope this helps and good luck
John