First you should understand that 50A is 2 50A 120V circuits for 100A total plus 240V capable and most RVs don't have 240V appliances. It's referred to as 50A 120/240V 3 pole 4 wire 50HZ.
The Cheater Cord as it's known will at most supply 2 30A 120V circuits which is a lot of power, but not 50A worth. And the 2 plugs may/may not be wired for 240V capable.
The Cheater Cord is often sold for 30A+20A plugs. The 20A plug is often a GFCI and will immediately trip.
The story is much longer and there are many posts on this subject. The Cheater Cord does work in some cases but it's not without issues and safety concerns.