Back of fridge if a test point is available. If you have a propane generator it should be running while testing as well as the propane furnace.
What my RV repair guy has always told me and has done on ours....
Bob & Sharon
2019 Winnebago Travato K (2018 Chassis)
Past RV's: 2013 WGO ERA 70A, Chevy PW Lexor
Itasca Navion, 29' Jayco 5th Wheel