[VAL] Dump valve removal

[Glyn Judson <glynjudson@xxxxxxxxxxxx>]

    All,

    It's a sunny and warm day here in SoCal and it's a good thing for that
because I've spent the last while this morning on my back under the Caravel.

    I need to remove the holding tank so as to access a much needed fresh
water plumbing leak.  The toilet is off, the flange under it is off but I
now need to remove the Thetford E-Z-Slide dump valve in order to slip the
holding tank sideways and out the back access door.

    Here's the problem.  From what I can see on the underside of the holding
tank, it appears for all the world that there are four incredibly rusted
mounting screws holding the valve to the underside of the tank.  This defies
logic though as am almost certain that those screws don't go directly into
the polyethylene tank bottom.  That said, the one screw I've tried to loosen
so far seems to just spin in place without backing out.  What's the secret
to how these valves are attached and how is one to best remove them from the
tank?  

    On another note, all the ABS from other areas of the trailer and to the
valve body below the slider is hard plumbed in place with no room for
cutting it and reinstalling with a no-hub coupling.  Is it standard shop
practice to sacrifice all that plumbing by cutting it out with a hacksaw,
only to replace it all with new parts?

    If I don't cut it out and somehow get the valve separated from the
bottom of the tank, I can simply reattach the valve when putting the head
system back together after the fresh water copper repair.

    Thoughts and helpful hints anyone?  So help me Gerald, if you tell me
that it's all explained in the McMaster-Carr Catalog.........  ; - )

    Thanks once again for your anticipated help,

    Glyn Judson
    1969 Caravel #508
    Santa Monica CA