Your Powerline EMS monitor bases it's 30A vs 50A status on the voltage between the two "hot" wires (L1 & L2) in the shore power cord. If the difference is 240v, it is 50A/240v service. If 0v, it is 30A/120v service.
You can have +120v on each leg and plenty of power, but the Powerline EMS will set 50A status only if the two legs are out-of-phase and thus representing standard 50A service. Would have to investigate the shore sources you used to know fr sure, but some parks don't deliver full 50A/120v service on their 50A outlets and many coaches still work fine with it. It would be odd, though, if you have been to several different parks and none of them provided standard 50A. I would begin to suspect a flaw in the Powerline.
Just to be sure, let me ask you to verify that you are indeed plugging into a 4-lot 50A outlet (using the 4-pin shore power plug) and not using an adapter on the shore cord to plug to a 3-slot outlet. A 50A cord using an adapter to plug to a 30A outlet will always show 30A on the Powerline EMS.