I have bought a 50a to 15a converter to allow me to simply plug in a everyday extension cord, i have basically the same thing for our TT.. I would like to take advantage of my garage being wired with 50a service though.
You really don't want to use that 3 prong outlet for your RV. It is a 240v 50A outlet with no neutral. The 3 wires are 2 hots and a ground. For an RV you need a 4 wire connection with a separate neutral and ground. There shouldn't be a ready made adapter. It is possible to use the ground for the neutral and make it work but it's not advisable at all. It could pose a rather large safety hazard.
By the way, 50 amp RV outlets are not 50A 120v; they are 50A 240/120v split phase.
That outlet looks like an air conditioner outlet. Go to Lowe's or Home Depot and get an RV50 outlet to replace the one in your garage. Verify it is actually 50 amp and 120 volts.
2022 Keystone Cougar 24RDS 2023 Ram 6.7L Laramie Mega Cab