Look what I found on AliExpress
Phone Gsm Remote control + (5KW 12V) water liquid heater for diesel /gas engine of car truck van bus! Webasto heater water type!-in RV Parts & Accessories from Automobiles & Motorcycles on Aliexpress.com | Alibaba Group
Just saw this. The gsm activation looks interesting. Running it eith used motor oil and waste vegetable oil could be profitable.
The price is a bit steep...
I wonder if there is a diy version?
Or maybe a very small generator that charges your battery, heats your coolant with exhaust and keeps the cabin fan running could be developed. While you are burning fuel might as well get electricity out of it.