Try this, prior to cranking on the shut off knob on that tank.

Soak it in Kroil, or any other pentetrating oil for a couple of days
before you snap it off.

Or, if your worried about any gas leaking  in  your garage after you do
snap it off, why not pull the Van out and just crack the feed line on
the valve and let the gas go to the God's?

Try soaking it first.
It may ot work, and it ay snap off, but at least you gave it a shot.

RC

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Subject: Forcing a rusty propane tank knob

Hello,
I have a rusty old propane tank on my 88' Westy. It is bad enough that I
can not turn the knob to shut off propane to the frig/stove. I really
want to shut off the propane so I can remove the frig and clean it. So
I'm wondering, if I take a wrench to the rusty knob on the propane tank,
what are the chances it will snap off and fill my garage with propane?
I'm also trying to burn off all the propane, but after 8 hours of
running the stove, I'm getting inpatient.
Thanks!
Matt