While I won't be doing toppers given the issues the camping buddies have with theirs flapping, etc...
and while the outside edge to outside edge of the slide will probably be enough for him,
be sure and measure everything and include a picture
outside edge to outside edge,
inside frame width ,
height from top of slide to lowest point of side at roofline
(because you want it has slanted as possible)
and depth when opened...
just to make it one trip
good luck !