I put a MaxxFan in my Boler
B1700RGH. The minimum roof thickness requirement means that to mount nicely there needs to be a spacer added, either on top of the roof, or under it; I used a two-layer plywood spacer under it.
Since the mounting holes did not line up with those for the original vent (there were not even the same number), I drilled new holes. The basic 14" square opening was fine for the new vent.
One nice feature of the MaxxFan is that you install a mounting collar, then drop the fan unit into it. Since it is difficult to reach in to the centre of the wide-body B1700, while standing on a ladder due to the roof height, I did most of the preparation (cleaning off old sealant) and installation (new holes, placing collar and fastening it) while standing on a stool in the trailer with my body sticking up through the vent hole - right through the collar while installing.
I can provide more details and photo illustration if that is desired; it will take me some time to prepare (select, crop, resize) the images.