I'd suggest you remove and clean all battery terminals and check all ground connections. Dirty contacts could be the cause of the fluctuating readings. Also I'd recommend testing specific gravity in each cell to see if you have any bad cells or batteries. If you turn on 12 v lights in the coach and disconnect shore power, do the lights dim? When you plug in again do they brighten? That will tell you if the converter is supplying power or not. Converter might be connected to the 50 amp leg that is reading 0 amps.
The fact that one leg of the 50 amp service reads 0 amps could be that the plug is bad, the pedestal is bad, or you have a circuit breaker tripped. It also could be that you only have electrical items on that draw from that one leg. A quick test would be to turn on second A/C which should be wired to the other leg. A multimeter to test the hot and neutral wiring in the pedestal would also be a good test. Each hot & neutral should test 120 v, testing across hot to hot should test 240 v. It's not unusual in hot weather when lots are drawing power to have voltage fluctuate, your readings are not out of range.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI