I replaced the front roof vent in my Boler a couple of years ago. Here's the story...

Removal of the old vent

The existing vent may not have been original, but it was in the original style. It worked, but was very noisy, the small fan was ineffective, and it couldn't be left open in rain.
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I don't have any photos of the orignal vent installation on the inside, but removing four screws allowed the inside trim to pull off.

On the outside, I scraped off piles of various sealants, removed the self-tapping screws which held the vent flange to the fiberglass, and pulled it off.
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The next job, of course, was to clean up the area, inside and out. On the inside, there was a set of four plywood strips as some sort of spacer, which were delaminating and garbage. Some of the screws even forced the spacer down, splitting it; I suppose they were supposed to be screwed into the spacer.
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Preparging for installation of the new vent

I test-fit the MaxxFan's mounting flange in the opening, and realized that even the four corner screw holes would not line up. I used a couple of them, and re-drilled the rest.
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From the inside, the test-fit with the whole vent in place showed that it would protrude as expected, and gave me a chance to look at how the spacer might fit in.
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The wide flange on the MaxxFan would hang out over the front of the raised mounting area, so I positioned it as far back as practical in the existing opening.
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I measured the protrusion of the inside face of the MaxxFan from face of the mounting flange, which will be above the outside surface of the roof, by the thickness of the sealant. The first photo is the assembled vent, complete except for the inside trim, sitting upside down with a ruler in place. The second photo is a closer detail; the vent is still upside-down but I rotated the photo to show it the way it sits in the trailer.
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Installing the new vent

The vent area in my B1700 is sloped and curved, and shaped to provide a level and flat mounting area for the vent, with very little space around the opening in the raised area. I needed spacers move the interior trim down to fit the fan properly, and to fit the spacers into this shape, the one closer to the roof is deliberately smaller; with a simpler roof shape, a simple single spacer of the desired thickness would work fine. Both are plywood, of different thicknesses. The photos show the MaxxFan mounting flange with the two spacers, then the spacers stacked on the flange, as they will be when mounted (except that they're updside down, and of course the roof is missing from the stack!)
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I ran two side-by-side strips of butyl putty on the flange, and placed it into the hole. After bolting it down, I removed the extra putty which squeezed out.

Rather than screw into the fiberglass (like the original installation) or even into the wood spacer (which would be better), I embedded tee nuts into the lower spacer, and used machine screws (not self-tapping screws). The screws are stainless steel, but as I recall the tee nuts are not. This is just a detail from the previous photo:
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And here it is at last: the flange is screwed into the spacers, and the fan is dropped into the flange.
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Finished installation

The interior trim goes on with some screws, in my case into the lower plywood spacer. I pinned the interior lining material between the spacer and MaxxFan trim piece; it was more than flexible enough to make the transition. This is the soft vinyl-faced polyfoam typically used in 17' Bolers; other lining materials might not be soft enough to do this. I made the spacers thick enough that the knobs don't protrude; they could be thinner (leaving the trim higher) if desired. The second image shows the surrounding area - you can see how it fits into the curves of the ceiling.
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From the outside, at a point of view at the same height as the vent, here are the final results, open and closed. The rear vent (replacement, but stock style) is visible for comparison. Yes, the roof does sag in the middle - the mounting areas for the vents would be level if not for the sag.
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