I battled the same thing and had to tackle it prior to moving up to Upper Michigan in late March for job. Growing up in northern Wisconsin I knew what to expect.
One problem with coaches is that, at least on my coach, the 2 rails on the main frame are open front to back. There are false ceilings in the 2 larger bays that push the air to the the wet bay area while driving. I took some time to use some spray foam to seal up the openings in the front and back, which helped reduce air infiltration. Not perfect but helped.
I have a small 300 watt heater in my wet bay that keeps that area warm but doesn't do much for the rest of the basement.
My water pump and water manifold are in the small basement compartment forward of the wet bay, the only heat there is if you run the furnace but it will do a marginal job. The intake for the water pump runs on the floor of this bay with no real heat source. There is a set of sliding doors that close off this compartment but it is open to the top so heat will migrate up and into the area between the rails. I ended up using a small cube heater firmly mounted to a heavy tool box pointed into the water pump bay leaving the sliding doors open. I didn't want to rely on the thermostat on the cube heater so I used one of these https://www.lowes.com/pd/EasyHeat-Fr...roller/1060249
on at 39F and off at 54F and plugged it into the receptacle for a TV near the outside wall of the compartment which would be a cold spot in the basement.
To keep track of the temp I mounted a remote temp sensor on the back side of the water tanks opposite of the wet bay heater, figured this would be the coldest spot in the basement. I had the monitor in the living are and at night I'd put it next to my bed.
I made sure the basement compartment seals were all in good condition, my basement doors do have an inner skin with some foam insulation in the door, not much but better then nothing. If I knew I was going to be in cold weather I drill the pop rivets out that hold the skin on and do a better job of insulating the doors.
The campground that I stayed at in the UP, although advertise as winter camping, was not that great but choices were limited. When I checked in they said I couldn't run any electric heat, I told them I'd pay them $100 more a month so I could run electric heat. They did not have any 50 and/or true 30 amp hookups so I ran one my coach power to a one plug and another extension cord to a receptacle in the bath house to give me 2 circuits. I ran one cube heater in the basement and one in the LR. I only used the propane sparingly as there wasn't a convenient place to get it refilled and didn't want to rely on it. I'd leave it set to run on the thermostat, turn the thermostat off at night and reach up and flip it on in the AM to warm the coach up before I left for work. Sometimes it would be in the low 30's in the coach in the AM.
I had a couple large sunshades in the front window draped with heavy blankets to try and help heat loss. Kept the shades pulled most of the time and the slides in.
Didn't hook up to water and they didn't have an open dump until May. I lasted all of April without dumping, taking showers in the bath house, washing dishes in the laundry room sink. (ah memories).
During the month of April we got +2' of snow and temperatures where usually in high signal digits in the AM. I survived, I think, luckily my wife stayed in TN until July.
I then rented a ski chalet at Indianhead starting in Oct and the wife took the coach back to TN for the winter and came back in the Spring of 2012. For the 2012 season they got 223 inches of snow. I talked my boss into letting me move back to TN in Jan 2013, by then I was traveling internationally spending +250 days a year on the road and the UP is not an easy place to get to. There was 4' of snow on the front deck when I left and had to have a plow truck pull me up to the house to load the Uhaul so I could pack it up and leave.