The micro switch idea is just like what happens with surge brakes on a boat trailer, the brakes would be on whenever the coach is decelerating, as when the jake brake is on. The fore-aft movement of a hitch over a ball is very small, less than 1/4" total is my guess.
Rick, your best bet may be to fine tune your Prodigy so that your brakes will come on when needed, but not before. Might take some trial and error, but you should be able to dial it in.
I've towed boat trailers with drum type surge brakes, then after switching to disc brakes (elect over hyd) on our previous 5ver, I put disc brakes and a new surge brake actuator on our boat (19' runabout, single axle). Very smooth.
I would swap out the drum brakes to disc on the boat if you haven't already. I wouldn't worry about overheating disc brakes, at least I have yet to do that. By the way, the steepest grade we saw last summer in the Rockies was Teton Pass, near Jackson, at 10%. Jake brake pulled her down nicely!. I wasn't towing a triple axle boat, however.