When I ran a Surepower unit I just got rid of the start assist feature. My belief is that it just mask a starter battery going bad. Wiring it to a switch like Brian did would be a great option, especially as you have the unidirectional model. You could use the switch to start assist to force closure when you were plugged in and thus charge up start battery if it sits for a while.
I am a little anal about my dislike for the start assist function and how sportsmobile seems to get it miswired so often. It was one of the reasons that my new 2004 rig spent about 3 out of a 8 week trip at a ford dealer.
In general Sportsmobile is not good a fusing at the battery (as in they don't usually do it) It can also be difficult to pick a adequate size fuse, that will still allow you to use it as a jump start option. The fuse that they added on your transit would be better under the seat with the battery.