[A/S] What caulk to use for a 1979 shower stall?

["Christopher45103" <blickcd@xxxxxxx>]

I have a 1979 31' center bath.  The shower stall has a built-in seat 
that goes up over the wheel well.  The seat and floor are all one 
molded plastic unit, while the walls seem to be masonite panels with 
a waterproof contact paper on them.  Lovely 70's flowers on the 
contact paper, might I add.

I have some leaks where the masonite panels are joined at the corners 
with metal channels and where they overlab the shower base.  This 
overlap is covered with a metal channel which has the usual plastic 
insert.

There is also a stepped plastic(?) piece where the outside wall 
curves down and has to "step" over the heating duct and transition 
into the seat of the shower base.

Most of my leaks seem to be at the end of this step piece, although I 
have caulked around all trim to be sure.  I have tried household 
bathtub caulk, and some white butyl caulk.  All hold up for a month 
or two, but after bumping down the road for a few hours and enduring 
drastic temperatures while stored, the stuff cracks and my leak 
returns.

FYI, all plumbing is visible when the access panel under the sink is 
removed so I know this is not a water line or drain line leak.  Not 
rain leaking in either.  Not much drips back in this space during a 
normal navy shower, but I want to seal it up to prevent any long term 
damage.

For those who have dealt with this before, what is the best solution?

Thanks,
Christopher