When connected to either shore power or generator the batteries will be charged by the on-board converter. This converts 120 volts to 12 volts for charging of the house battery(s).
For the converter to work it only requires 120 volts in. This 120 volts should go through a circuit breaker so that would be the first place to check. You can cycle the breaker off/on to see if that helps.
If the converter is working you should measure mid 13s to mid 14s at the battery. If you are not then maybe the converter has failed. Find the converter and verify that 120 volts is going in. Look for any fuses on the converter and check them if present. Measure the 12 volts coming out of the converter at the converter. Should be at least in the mid 13s. If nothing is coming out but 120 volts is going in, the converter is bad. If mid 13s comes out but you don't read that at your battery, look for a break in the connection between the two.
I have attached the troubleshooting guide from my converter.