When I had to adjust mine, I referred to the Lippert video citgodan linked. Of course, the one thing it doesn't cover is the concern you have over the top seal not seating. A quick assumption is that you can correct that with the vertical adjustment if you find there is much more than the 1/4" difference. If there isn't, you'll need to look further at the unit to see if there is another issue - such as rotten floor where the outer arm supports attach, though that is not very likely.
In my case, I had too little room at the top, it was so high that the seal was being pinched by the screws on the horizontal flange (which are covered by roof tape) and wearing the seal and the roof tape. I was able to lower it enough to get clearance, though the dimensions in the Lippert video did not really correspond with my clearance problem. I suspect the Lippert video assumes the slide room is manufactured to the exact correct dimensions, which may not always be the case.
These things are always a bit tricky. Take your time to evaluate as much as you can before you make a move to purchase this unit. Ask the owner a lot of questions and look at everything you can. It looks to me like it is not a big problem, but I am not able to see the big picture.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS, Progressive Industries hardwired 50A EMS HW-50C)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels