You can buy the conversion kit from a coupe of places... ETRAILER.COM and AMAZON has them. CAMPING WORLD as well...
Just be sure when looking on google to make sure you are looking at a 50A kit. They has the same kits for the 30A conversion as well...
If you are not an electrical type I would would go back back the RV DEALER and have them make the conversion for you.
This is not a good idea for your brother-inlaw or father-inlaw or friend to do the conversion for you. This must be wired for RV's and if wired like the house circuits etc you run the risk of blowing up a bunch of 120VAC appliances. Kinda hard to collect moneys from the family and friends. The commercial guys are insurance bonded for these things and should pay for what they blow up wiring it up wrong...
lots of info on-line on how to do it....
Etrailer probably has a video of how their kit gets installed...
Roy Ken