Problem with 50 amp cord

Is it really the cord? Or one leg of your breaker? Start there.

If not one leg on the breaker box then use a meter to verify correct voltages at pedestal. Once stayed at a campground that offered 50 amp but when metered only had voltage on one leg.

Seems to me that it MUST be the cord. Everything works without the extension. Everything works when I run my generator.

if i read your post right you have normal power all the time *except* when using the extension cord? if that's the case then there would seem to be a problem with that cord. i'd start by running a continuity check on that cord using a volt-ohm meter. sounds like the wiring forcone leg of the new cord is not connected and the continuity check should confirm that.

If you own an RV you must on a Multi-meter and know how to use it. Youtube is your friend.

Sounds like they messed up the connections at either the plug or outlet end of the cord. Now as it happens I have a way to tell which. but it is a product designed for other use (Antenna Analyzer) but the only way I know of to fix the cord is to replace the faulty end. Problem is deciding which end it is.
Oh a multi meter that reads capacitance may do it too
The good end if you measure legs to ground or legs to neutral will have a higher capacitance but I don't know if it will be enough to notice. The bad end the two legs will show different capacitance.

I would contact the supplier of the extension cord. Maybe they have a known problem and will take care of you even if has been a while. Otherwise if the connectors are not molded on you can remove check for a disconnected wire.

If this is a detachable cord, I would check the connections in the twist lock end. It is more likely to be bad connection then a bad cord. Ends are available.

From your description it does sound like the extension cord. Sounds like either a broken wire or a bad wire connection in one of the connector ends. If no visible damage then likely a manufacturing defect. If it has molded connectors on each end, then the only repair option is to cut those off and replace with new connectors.

Are you positive it is a 50 amp extension cord? Does it have 4 prongs and 4 female inserts? I think you have answered your question---The cord is defective. Doug