There is no need to disconnect 12V to reset.........
Your Suburban WH has separate controls for electric heat & propane heat
Turn ALL switches OFF
In outside compartment above gas valve is a rubber cover with 2 buttons....Push to Reset
Left side is for electric. Right side is for propane.
Those are the separate set of t-stats. The reset is to rest the high temp t-stat
The electric uses 120V AC power....comes from a AC circuit breaker. Check if it is tripped
The propane uses 12V DC....comes from DC dist. Panel....check if fuse is blown
After resetting, checking circuit breaker and fuse, check valving on backside of WH (access from inside RV) and make sure you have cold water valved OPEN, bypass valve CLOSED and hot open (IF there is a hot out valve).
Also check showers (inside/outside) that they are valved out at the cold/hot knobs and not just with button on handle.
Then check that you have WH full of water (Open the relief valve in outside compartment with lever and then let it snap closed----CAUTION water could be hot and will spray)
Then turn ON the propane switch (inside panel with light)
Listen for spark electrode (click, click) and main flame lighting off
If it does light let it heat up until main flame shuts down. Roughly 20 minutes
Got Hot water from faucets?
Then turn gas switch off....use the hot water until it cools down
Turn on electric heat via switch in outside compartment.
Let it heat for 30 minutes.
Got hot water?
Report back with any problems.......and what isn't or is happening. OK!