Thing to do is turn all other electric off and then check what your voltage is at any plug. Then run the AC and then check the voltage at any plug. Even at 50', I bet you drop below 108v... While the AC is running, that low of voltage isn't doing your motor any favors, so...
For running anything else in the rig, you should be fine. It's the AC that will always tax the system to it's max, so gotta decide what you need to run and go from there.
I installed a dedicated 30a RV plug because when I ran my AC, it dropped my voltage to around 108v with only a 25' cheapie extension cord. Sure, I could have gotten a better cord, but decided to just go with the 30a setup..
Good luck!
Mitch