That rubber boot originally went to the spark electrode on burner (someone changed out...see red wire attached inside burner tube)
You stated you had it running..lit off, good blue flame and then it shutdown...
Did it run long enough to get water warm?
See if t-stat has tripped (reset by pushing in on rubber buttons).
Is this an LP only water heater...no electric?
Need good 12V DC power and good grounds. So clean tighten any ground wires and check that connections are clean tight on power wires.
Your ON/OFF switch may be located next to monitor panel BUT the control board & wiring junction box for water heater is on the side of water heater.....access from inside rig.
You need to locate them and check wiring and also fr 12V DC power.
Pic's can only be uploaded from a public photo website (like tinypics).
Using the 'advanced post form' click on yellow icon that looks like mountains with sun (image)and then paste link from photo site.