We have winter camped often. It can be very enjoyable, no bugs, no crowds and an entire State Park to yourself or just your neighbor who has the camping itch as bad as you ....
To the water situation, your camper right now should be winterized. Meaning all water is drained out of it, blown out with compressed air OR drained and food grade antifeeze put in the lines AND the sink and shower traps. Any water left unprotected in the system will freeze and odds are favorable to crack the piping. Do not know if your prior owner has done this, do you know? If you need help winterizing let us know we can help and have many articles on winterizing. They really cannot survive filed water left in the system. The freeze damage can be very unforgiving.
Now to camping, I have customized our camper special to be able to wet camp but this is a bit more involved. For your first time out, I would say, no do not go wet until you have figured this all out and taken special precautions to not crack a tank, drain pipe or your inside camper piping. Most Sunlines where not made for deep cold weather camping in their stock state. Some have had upgrades and heated tank systems, but again they are specials.
For many years we would use the camper dry, find a SP with a heated shower house near by and this provided a heated potty. In some cases some parks only had a outdoor porta potty and that worked too.
Inside the camper we put a dish tub in the sink to wash dishes and a small rubber maid bowl in the bath sink for teeth brushing and face washing etc. Then take the water out and dump the water. A 3 gallon fresh water container did us all weekend long.
The next thing to know is, inside condensation. Running the heat inside and you and maybe your partner will create moisture in the camper. And the water washing etc makes this even worse. You are going to have to deal with excess moisture or you will wake up to totally water sweating walls, ceilings etc. Not to fear, just educate how to deal with this.
One method is using a dehumidifier. This is the advanced end of winter camping. It takes time to sort out the right size, does not make so much noise it does not drive you nuts... etc.
The other method is venting. Yes, it is 15 degree outside and your opening a vent in the camper... The moisture is trapped in the air inside and then collects on walls, windows, bedding etc. It seems the worst during sleeping. While washing, cooking, crack a window open near the water source to let the heavy moisture load go right out side.
We open the roof vent the furthest from the bedroom and have to open it a full 1" on the opening end when sleeping. Yes, heat is going to go out that hole... and so is the moisture. The moisture is dying to get out, so let it go...
Your windows will fog up and be wet. Wiping them, having a fan blow on them or using window shrink wrap to make storm windows is about the only way out of the windows. You can control wall and cieling sweat with roof/window venting. If you have a lot of moisture, then a 2nd side window cracked open to force a slight cross air path helps.
Other then taking care of the moisture, the rest is just go have a good time.
Hope this helps and happy camping.