Just a guess/you didn't post BRAND
You probably have a Suburban Water Heater
You have a direct short that is blowing that 15A fuse
Either in DC wire to the On/Off Switch
The On/Off Switch contacts
Backside of Water Heater tank is a DC Junction Box
DC wire from On/Off Switch goes there and is wire nutted to water heater OEM wiring so could be a bad connection.
If NOT in the DC Junction Box then need to check the DC wire to the LP Set of t-stats
*Under the cover above gas valve...right set of t-stats/red wires.....sometimes the wire will rub against the sharp edge where opening is punched out for the t-stats.