Interestingly enough, just this morning my son and I replaced both toppers on my rig.
ToughTop provided the new toppers.
Turns out to be a piece of cake with two people (44 & 72 if that is important to you
) and two eight foot a-frame ladders.
To your point, one of the installation check points is to measure your slide depth fwd and your slide depth aft to ensure equal for a nice snug fit. Slides can get canted a tad, and it wouldn't take much to cause sag and/or uneven uptake when rolling in. If it is canted a tad, your owners manual should describe how to adjust. For me, it is the same process as manual retraction. YouTube has several instructional videos also.
Fair Winds and Following Seas!