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.
Good Luck
Fair Winds and Following Seas!