With 120V charging, it is also important to check the receptacle. Turn off the circuit breaker and unscrew, then inspect the receptacle and its wiring. If the wires are inserted via 'back stab' holes that use little knife edges to make contact, rewire using the screw terminals. You have to insert a very small blade into a slot to release the knife edge allowing the conductor to be removed. If the receptacle is commercial grade, the wire will be clamped by the screws in any case.
And speaking of commercial grade receptacles, homes are often full of el-cheapo 'residential grade' receptacles that come in 10-packs for like five dollars. When I first bought my house, I went through every switch and receptacle and replaced most of them. Some older ones were literally crumbling as I removed them. Commercial grade receptacles are not very expensive and far better constructed than residential grade.
As far as extensions go, I've both made my own - using SJOOW cable, and commercial grade plugs and receptacles, and also have a commercially made one for TT-30. With the TT-30, there are lots of off the shelf adapters in the RV world to go between 5-15/5-20 to TT-30 and back. On road trips I just carry the TT-30 extension with the adapters in case of emergency.