Appropriate float voltage for SLA battery in 120°F+ environment?
According to Battery University:


  The recommended compensation is a 3mV drop per cell for every degree Celsius rise in temperature. If the float voltage is set to 2.30V/cell at 25°C (77°F), the voltage should read 2.27V/cell at 35°C (95°F). Going colder, the voltage should be 2.33V/cell at 15°C (59°F). These 10°C adjustments represent 30mV change.


So, for a 30°C difference in temp from 25°C (55°C is 131°F), I'd have 90mV per cell of compensation, multiplied by 6 cells to reach a temperature compensation of 0.54V for the battery.

So, instead of 13.8V float, I should be floating at 13.26V or less. 

As a result, I've modified my float voltage to a safe 13.1V just to leave some additional room / buffer.