If you are running an extension cord from your house 120VAC receptacle it should be a 10 GAUGE (10-3) HD duty extension cord (you may be able to use a 12-GAUGE 12-3 - DO nOT go smaller than 12-gauge). Then you must have a RV30A-15A Adapter on the end of this extension cord to plug your trailer 30AMP shore power cable into it with. This adapter must be one the long dog-bone type cable. When i used the small round one they always go hot on me.
You need to use a good AC VOLTMETER and ,measure the 120VAC receptacle at the house. The measure again inside the trailer when you are connected up.
This NO SHOCK ZONE photo is good one to follow on how to measure your 15/20AMP receptacle in the house.
This is the RV 120VAC meter I use to plug into a receptacle inside the trailer. This meter has a SAFE ZONE painted in GREEN. If the AC VOLTAGE is outside this safe zone I will not turn on my trailer air conditioner
This is what I do to watch my 120VAC power coming from my garage...
You DO NOT want the connections to get hot and burn up your plugs like this photo shows...
Just my thoughts here