I took a good look at your MH construction links.
I think that you would be "OK" with a ground that catches the metal wall / roof supports. This would essentially tie to enough metal to allow for a fair capacity ground system to work, especially given the 40 ft lenght you have.
The Flagbuddy mounting system would also work well, you would have to use a bit stronger mast tham the FlagBuddy system but their mounts would work very well.
I saw some aluminum ribbed masts on eBay that would work to get the antenns coil of a screwdriver above the roof line. http://www.ebay.com/itm/380383629712?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
I would tie the ground to the sidewall internal frame members as close the bottom of the antenna
itself as possible using a good 1" tinned braid.
Again, Alan (K0BG) has some really important information on interaction between the screwdriver antenna and the controller wires that would be a good read. www.K0BG.com