Suburban water heater gas valve stays open no flame

Why are you so sure that there is no other leak and the main valve is not shutting off correctly?
There is some form of solenoid that cuts off the gas flow when no flame is present.
There is some form of thermocouple that senses the heat.
The system could fail in a number of ways mechanically (stuck solenoid, loose connection, for instance) - the control board would not be my first suspect.
Propane can be real dangerous. Take care, and take it to a professional if you are bot sure what you are doing.