Originally Posted by HarryStone
Red and black should be power inputs, white is neutral. So white to white should be 0.
And because you (the OP) mention L1 and L2, we will assume that you have standard 50 amp RV shore power.
Outer straights are the hots (L1 and L2)
Center straight is neutral
Center round is ground.
So from either outer straight to either center round or center straight= 120 VAC
Outer straight to outer straight= 240 VAC
Center round to center straight= 0 VAC
Let us know what you find.