Seein gas you have a Suburban Furnace then you would have a Suburban water heater (vs an Atwood)
Suburban WH has an ON/OFF Switch for Electric Element in outside compartment lower left corner
On/Off Switch inside is for the Propane
Fill WH Tank........open hotside of any faucet to bleed air out of system
When tank/hot system is filled (air bleed out of line)
You can turn electric on via outside switch provided you have 120V AC Power
You can turn on propane via inside switch provided you have propane and 12V DC Power
You can turn them both on for 'fastest' recovery