Interior skin access problem

[HolyRoller]

I am addressing the rear end separation and some floor rot in my 1968 safari. Bathroom has been gutted and plumbing removed. I am trying to gain access to the C channel behind the interior skins on the curb and roadside (I have accessed the area below the rear compartment door).

My problem is as follows. There is a (PVC/HDPE?) upper cap that overlaps the lower skins by 4"-6". The interior bathroom wall/divider covers the vertically riveted end seems. As such I unable to fully remove the lower skins (due to lack of access to the vertical rivet strip) or peal them back towards the tongue (because of the ceiling cap).

Any ideas? I sure hope I'm missing something as I would like to keep the disruption to a minimum.

Thanks in advance for any and all of your time!