If you are serious about heating a trailer, get a vented RV heater. I have built 3 fish house trailers using them. ( I used Suburban brand) can get them from 12–40k BTU. Very simple to install and use. More $ than the diesel heater but you get a “set it and forget it” t-stat. It’s forced air so you can run ducts where you want them.
BTW you guys building out cargo trailers should research diy fish house builds. Several years ago there was a lot of discussion on this and a lot of good ideas and pictures.
Mnbill