IF you are connected to AC Power Source then turn on the electric and the propane at same time.
That 6 gallon will heat to 130*F -----electric/propane shut off
Then turn back on when water temp drops to 110*F
How long you get hot water is dependent on how HOT of a shower you take.
130*F will scalded within just a few seconds so you are not using straight hot water.....using it tempered with cold
Residential shower avg temp is 105*F, lasts 8 minutes and uses 40 gallons of water
You are NOT going to get that with your RV water heater
BUT you can take a long warm shower for several minutes and extend the supply of hot by using electric/propane and NOT letting shower run the whole time you are in the stall.
That Suburban WH is rated at 20 gallons per hour recovery rate using both heat sources