Sorry you are having issues with getting HOT water
Your Suburban sw6de does not have a 'thermocople'
It has a Spark Electrode that provides High Voltage for Ignition AND return pathway for the Flame proving Signal
*Large Orange wire
But first let's check a few basics
When on Propane do you get Spark (CLICK, CLICK, CLICK) and gas valve open??
And Flame at all?
Do you have AC Power......
Does water heater work on AC/Heats water OK?
Propane uses 12VDC
With GAS Switch ON.....check that you have DC to/thru the set of T-stats
*Located above the Gas Valve ----under the 'Push To Reset' cover
*Right set Red wire is the 12VDC t-stats for GAS
Top is the HI Temp T-stat...Push to Reset when it trips (170*F)
Wire goes down from HI Temp to Normal Temp T-stat (Opens at 130*F)
*Wire can melt if amp draw too high
IF you have to Reset the HI temp then it is time to replace the SET cause the Normal T-stat Failed
If Hi is not tripped and wire is intact then issue COULD be with the Circuit Board
Circuit Board is on the BACKSIDE of WH Tank...or adjacent wall
Here is the 12VDC Wiring for your water heater GAS circuit