You have to pull the circuit board cover to get to the 110V wires going to the heater element. You have to pull the two wires from the board to check the resistance or to put 120V to it BUT once the cover is off put it back on AC and check how much voltage your have on the heater element wires. If you have 110+V then unplug from AC and remover the wires from the board and check the resistance...44+/- ohms. If more than 60 ohms I'd say the heating element is going bad.
Doubt your 8 CU Ft refer has a rear fan, but if it does, are you sure it's working? If it is in a slide out then you may have a fan that's not working.