Do you have a toad, and close to a RV dealer or Camping World? Many of them will carry a replacement latch like this, that looks like it would work on your door (but not sure without seeing how it's attached behind the access door). However a metal one could scratch the finish as opposed to plastic, which they may also carry:
Water Heater Door Camlocs, set of 2 - Camco 09213 - Water Heaters - Camping World
If worse comes to worse, you could just insert a nylon bolt (that would fit thru the round hole in the slotted part of the door) in place of the camlock from the backside and use the corresponding nylon nut and washer to secure it till you get home. This video shows how easy it is to remove the camlock.
I'm also pondering if you could use a small keyring in place of the broken plastic handle.