I have not used one of the older camper showers, so I may be off base on this.
I agree with your thoughts, if that wall papered wall is exposed to water, I myself would not trust 1988 year vinyl wall paper to be water proof. It may be, but I have had to deal with too much water damage repair so I am coming from that point of view.
The lumber yards do sell white (or clear) plastic sheets that you may be able to glue on and then seal up. If you can post some pics, of your setup, we may be able to think of some other ideas.
Hope this helps and let us know how this comes out.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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