This Spring has been very cold for May.
Camped at the 7800 elevation mesas of New Mexico where the mornings were in the mid-20's and at sunrise goes into the upper 60's / lower 70's for Solar Heat, but the air is still nippy. That was the first two weeks of May this year. Snow in Arizona mountains to the West.
After Sunrise... mid 70's, low humidity and pleasant clear skies.
Used five minutes of furnace the first day AM to get prepared for the 2nd Day.
Dog's water bowl outside froze to the bottom (about 1-2 inches of ice). Inside 10 to 15 degrees warmer. Say 28F outside... about 38F inside. The furnace was to just take the 'nip' out of the morning. After that first day, the following ten the furnace was not needed... at all. AGM Interstates did not go below 12.8v.
Soon after Sunrise, open windows and free heating all day... until Sunset. Repeat entire previous routines. Repeat, the next. Etc.
We use multiple covers and wear sweats. We have 160watts on the roof and 100 watt Costco portable Solar Panel directed to the Sunrise that recharges our batteries to 100%.
We have done this since 2006. The Furnace is great when hooked up to power at a RV Park. Out Boondocking... you have to be on top of performance and your personal comforts. We are prepared with bed coverings and covers for the two Blue Heelers. It is learned... and you will develop a system... or not.
The Airstream is not a 12 month home on wheels for comfort. There are plenty of posts on the Boondocking Threads how to develop a SYSTEM. Comfort at 25F outside and 38F inside is what to expect. Kill your batteries and run out of Propane it will be 25F out and inside...
Add a Little Buddy with the small expensive propane cylinders for taking the 'nip' off the cold. We survive. We get tough. Our Blue Heelers have no complaints. Our water system does not freeze up. These techniques are on the Boondocking threads.
We have survived 18F in Wyoming in July. Water lines froze in some areas and this was one of our first trips with the 2006 Safari. We learned how to prepare. You will as well. One step at a time. Eventually you will be on top of it and how to park the trailer for heat or for cold when Boondocking.
The option is Tent Camping. If they can survive without incident... you have a Trailer with many options. They do not. Tent Campers to an Airstream... problems have already been solved. That was our evolution. The learning curve was not as steep for us.
Good luck. You will get it figured out sooner than later.
We survived. You will as well.