That was something I was thinking about doing but am not finding instructions on how to do the Firmware upgrade. You have a link that shows instructions on how to get that done?
@jwt355 Here you go!! 
https://cdn.shopify.com/s/files/1/0536/3390/8911/files/How_to_upgrade_the_firmware_in_App.pdf?v=1669370094
I have the EB55 though so I dont think I can use the App. I tried to add my unit and it does not let me. If you know differently please let me know.
I posted this on Facebook also and Doc Hall Linked me to a video on youtube that provides the solution Just go buy one of these and this solves the problem Amazon.com Dont use the 12volt socket instead use this and plug it into the AC Port this will do the trick.
@jwt355 Ahh alright I totally thought you had the eb3a unit since the OP was talking about his EB3A. Yea the eb55 doesnt feature the ability to update its firmware, nor should you really need to. It is strange that your diesel heater is having issues with the eb55 cigports regulated output. You could try that ac-dc stepdown converter that you posted above, but will experience additional efficiency losses, but the eb55 is pretty darn efficient overall so shouldnt effect runtime that much! Make sure to keep us posted on your results!
The EB55 is still my “go-to” unit for day trips. I love that little beast!