Dielectric grease for 30 amp plug and receptacle?

Dielectric grease is used OEM on sensors operating in the millivolt range.

The only electronic contact cleaner that works is

DeOxit D-100

Don't play games with the D-100 part number. It is the strongest DeOxit cleaner and you need it. Try squirting it into the receptacle slots and then onto your plug prongs. Then slip the plug in and out 20 times to rub off the corrosion.

Without disassembly this is the only thing you can do. Other contact cleaners are a farce. Let us know if this helps.