What you have depicted in the linked file is 50 amp service with a female 50 amp receptacle and an adaptor with 50 amp male plug on one end and a 50 amp THREE wire female end. Again, just replacing the female three wire end with either a four prong 50 amp male or 30 amp male plug creates a "widow maker".
The previous owner may have wired it like this so that it could be connected at home to a three prong dryer or welder outlet but would have had to use a "widow maker" for the final connection. It is not "Typical RV connection to power pole" as your diagram is labeled. I would suggest to install a 50 amp power inlet on the 5th wheel and use the correct cords and adaptors.