No need to reinvent the wheel!!
Check with one of the many on-line towing and hitch stores like etrailer, hitchweb, etc.
If your motorhome is a Ford chassis measure the from outside edge to outside edge of your frame rails. Should be 33 & 3/4 inch to 34 inch. Same frame width as a Ford F550 or F650 stripped chassis. Order a hitch for the truck with the correct frame width and you're good to go.
If you have a different frame width then call one of the hitch dealers and give them your frame measurements. They will have a product for you.
I just ordered and received a 10000 Lbs capacity hitch made by CURT for $199.00. I would have got the 5000 lbs model but the 10000 lbs was on sale.
As for welding, not required. The new hitch bolts on, all hardware included. Remove old hitch, Clamp the new one in place, drill the holes, bolt it on, apply correct tourque, you're done!
Remember to not exceed the recommended towing capacity in your owners manual.
Why am I replacing my hitch? Well that question is worthy of a complete new post, but I need to get in the garage and start my hitch replacement.