I built a cross member with a center and two outer recievers. The center I use to tow a small flat bed for the golf car. The 2 outer recievers are for a motorcycle carrier that I modified. Works great. I have done this for other friends also that use it for a generator rack.
I'm trying to find a pic of the rack on the trailer but I don't know if I have one. I only have this one of when I just finished it and tried in in my truck.
If you have a local weld shop or fabricator ask for a quote.
I'll advise don't utilize the thin wall 4" square tube that is on the back of most rvs. It will not hold up. That cross member I built is 1/4" thick steel. The rack/ carrier started out as the carrier that CWSWINE posted a link to above.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS