there is typically a breaker at the hull
where the power cord attaches to the boat
....if you have two 30 amp cords you will have two breakers...it is possible that one tripped. It's a square looking box thing typically with a plastic panel over it. Lift
the cover up and you will see the breaker switch.
Switching you cables around does not identify this problem as you are still using the one breaker which may be operable.
Also check the " connectors"...these are the little metal teeth that stick out from the power cord...should be 3 of them....check for burn marks or melted rubber.....sometimes water
gets in there and can short this out...check the connection at the splitter and also at the boat.
An a/c, when starting up, can suck a lot of amps.....maybe more than the dock
power can provide....and trip the system.
Hope this provides some guidance.