According to info on the Keystone RV owners forum, the Suburban RV water heater thermostat should be set for 130 degrees, and is non-adjustable. It comes wired in series with a 180 degree secondary protection thermostat. There are 2 sets of thermostats on the Suburban RV water heaters, 1 set for 120 volts AC electric power and 1 set for 12 volt DC control of propane gas heating of the water.
BTW, are your cool water temperatures happening with both 120 volts AC power on the water heater and propane gas operation of the water heater?
These thermostats are pretty easy to replace under the black rubber cover on the outside section of the water heater.