The power cord is very hard to get to. I would suggest checking the transfer switch first. On my 3121 it was under the steps. Accessed from inside the storage compartment. Opened the compartment and the bulkhead panel easily unscrews getting you to the backside of the steps. I think you will find the transfer switch right there. Pull the cover and you can use you meter to measure incoming vs outgoing voltage. Mine had failed. It also failed on one leg like you describe. PD sent a new one no charge. They stopped making the one with the built in surge guard it was so prone to fail.
In a pinch you could wire across the transfer switch and eliminate it altogether.
Good luck. Hope yours is in that location. Pretty easy to access and work on.
2021 Ram 3500 DRW
2022 3231CK Legacy, Shadow FBP, Reese Goosebox