We also have gasoline engine & no propane here. I like the simplicity of all electric.
Usually, we're driving around during the day, and camping at night -- rather than staying at one spot long term. So, the van is usually still warm for a while after parking, then we cook dinner -- and the stove keeps the van warm. Finally, we sleep with a blanket and two down comforters (on freezing nights). In the morning, the van gets warm again by cooking breakfast.
If we do want to stay up late after dinner when it's cold outside, we use a catalytic heater such as the Coleman SportCat
or one slightly larger. These heaters can sometimes be a bit smelly.
This all works great down to the mid 20's degrees or so, below which we start having problems with pipes freezing overnight.
[edit: by cooking, I mean using a portable butane stove.]