2018 3500 EXT
A gasoline Webasto is an option. I'm also considered;If you are worried about heating your van, space heaters are great for when you get to hook up to shore power. But when boondocking that won't work for long before draining your battery bank. Webasto is gasoline powered (Ram Promaster even has a specific spot in the floor where you can tap into your gas tank!!!), takes fuel from the van's gas tank, and only uses a little electricity to run. It's a dry heat and all venting happens outside under your van. That's what I've seen others doing with great success, and what we will do when that time comes.
Inexpensive diesel heaters like the Webasto, but $200
and I settled for;
Propane Propex HS2800.
You will use more 12v on the gas & diesel than with the Propex (especially on startup). All these fuel types have pros & cons.