I found a detailed manual - not on Thor or Atwood site...
It appears that you may not want to replace the supplied one. The manual lists a delaying mechanism in the supplied thermostat that’s likely not available in an aftermarket one. Here’s the relevant page from the manual. I plan to call Atwood and ask specifically whether it can be replaced.
Here’s the link to the detailed manual that talks about all the functions...
http://bryantrv.com/docs2/docs/hflamefurn04.pdf