Sophie's choice: One way, you fry half the appliances in the RV... The other way, you electrify the chassis of the RV. Whoops! The only other possibility with a 6/2 cable is to hook up neutral and ground properly, then not hook up one of the hot wires, but this too is bad (just less destructively).
The only way to support a 14-50 with 6/2 is to add a 12 KVA transformer to the mix.
But why bother? You don't benefit from a 14-50. You benefit from a TT30, which is the plug your RV actually does have.
That's a much simpler matter.
Change the 50A 2-pole breaker to a 30A 2-pole* breaker.
Move the white wire from the breaker to the neutral bar. Very important!
Throw the NEMA 14-50 recep on the shelf. You will not be using it today.
Install a TT30 receptacle.
Enjoy your RV.
Now, you may find the breaker or recep will not accept a #6 wire. In that case you'll need to pigtail a #10 wire onto it. The big blue wire nuts should suffice. Crank Hard. No... HARD!
* Mind you, you don't actually need a 2-pole breaker, but you do need to fill the empty space, so a 2-pole breaker takes care of that. Also, 30A 1-pole breakers are essentially useless for any purpose but TT30 RV outlets. 30A 2-pole breakers are used for all sorts of things.
