There are several possibilities, but 50A vs 30A shore power is not one of them.
1. The electric heater element could be burned out or have a loose wire
2. The 120v power relay on the fridge circuit board could be failing
3. The 120vac fuse on the circuit board has blowm
4. The 120v branch circuit that powers the fridge in AC mode may have a tripped breaker
5. The outlet the fridge is plugged into could be bad.
#4 and #5 would prevent the fridge from automatically selecting AC mode.
The fridge controller uses 12vdc power for its operation, whether in LP or electric mode. The light comes on to indicate the controller is working and has selected, but does not mean that the AC heater is actually working.
What lights are on the display? Do any of them blink? For example, the Norcold N611 will blink the ON light 3 times every 3 seconds if the electric heater has failed. 4 blinks every 3 seconds means the control board is failing. The Fault Codes are covered in the fridge owner manual. If you don't have it, get it at http://www.thetford.com/wp-content/uploads/2015/04/1297/Owners-Manual-Through-32306.pdf