If it can wait to Monday I recommend calling Nils in Tech Support for guidance at (toll free) 866-293-9078 and dropping them an email at tech@arbusa.com.
The thermal fuse is recommended in the Engel (first generation ARB units) to protect the fridge unit. They are a bit pricy compared to a standard glass fuse but they are only available via import from Engel proper thus the added cost. One might be able to source an aftermarket thermal fuse of matching size but for the few dollars I've honestly never looked for anything other. I keep a spare fuse in the pouch on my transit bags (both on my personal unit and my rental fridges), while a blown fuse is rare, warm milk sucks