How are you measuring the voltage drop?
You should be using a volt meter at the pedestal.
There will never be a voltage drop if there is no current being drawn.
And as the current increases through a poor connection, the voltage drop increases.
The first thing to do is measure the 50amp voltages, then plug in your adapter and measure on the 30 amp plug.
Voltages can be +/-5% at the pedestal without a load.
You should feel significant resistance when plugging in your adapter if the contacts are good.
Finally a weak spring or corroded contact in the adapter can certainly cause a problem.
IF you have a cord reel, it also has contacts that can go bad.
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