I have not posted in this thread for quite some time but in light of numerous discussions about trailer hitches and how and where to mount them, I'd like to add my two cents worth.
My skoolie was built an a 2005 Chevy W3500 chassis (read Isuzu NPR). While studying the GM Upfitters manual for that chassis, I learned that no drilling, cutting or welding is to be done on the upper or lower flanges of the frame at any point whatsoever. With this in mind I designed and built my trailer hitch to bolt to the web (side) of the frame following the pattern mandated in that publication.
There are several strongly stated opinions as to whether or not it is OK to mount a trailer hitch to the bottom flange of a frame. That being said, I strongly recommend that each builder of a skoolie who plans on fabricating a trailer hitch determine via the mfg of the bus chassis exactly what they recommend as to mounting points and fasteners. As will be clearly seen in the following attachments, mounting a hitch on the flange of my frame would not be acceptable.
I apologize for not being able to get the attachments to rotate. They can, however be enlarged by left clicking on them. I sure find Windows 8.1 hard to use! Jack
This is a pic of my hitch prior to final welding and gusset instalation. The hitch bolts to side of frame and to the rear cross member.