Please don't give them any ideas. That 20 second MIDI track they play over and over again as loudly as possible is like fingernails on the chalkboard to my misophonia.
When I was a boy, there was a little hole in the wall ice cream spot... It was LITERALLY a window/counter in a wall. In the winter months, that window sold pizza slices.
This is the way.
Insulation is insulation, it can keep something hot as much as it can keep something cold. And a refrigeration system reversed becomes a heat pump. Would cost next to nothing to make something like this happen from a mechanical standpoint.
While that's true regarding insulation, the degradation of materials might be different at different temperatures. It might not be an issue as I have no idea what the materials in an ice cream truck are, but if you want to maintain a hot beverage-temperature, several types of paint, treatments, walls and floors might fail over time.
When I actually lived in cold climates, I almost always purchased my ice cream to consume during the winter months. During the summer months I was more likely to purchase a peach smoothie or some other summer fruit that I can't get outside of summer and early autumn.
My first thought was "hey, I'm down for that!" and churros and a ton of other things. But ultimately you're right. I'm still down for ice cream while it's snowing.
The ice cream truck here in IB, CA, US, runs every day of the year. They serve nachos on Saturday and Sunday.
Might be because we never have temperatures lower than 4° C overnight in February and early March, and we never see temperatures above 30° C at the peak of summer.
My regular Saturday morning walk along the Mersey had a hot chocolate van by a lake just before the final stretch. Often the ice cream van was also still there even when it was cold.