I said:
>I don't know whether there would be legal or certification issues,
>but as a practical matter I think you'd no longer be able to get
>filled by a propane truck, heating dealer or any other facility that
>does not refill DOT tanks.  You'd have to carry around an adapter to
>go from the larger fill hose fitting to your smaller one, and I

Andrew Martin pointed out I've got my dimensions wrong.  1 3/4" Acme
is the normal size of RV fill valves.

If you're replacing an Autostop valve it will be *absolutely*
necessary to add a manual vent valve at the 80% fill level fitting
that is piped to the autostop valve.  Without that it will be
impossible to fill the tank safely.  Your local RV place may have
them.  I haven't been able to find one online in ten minutes
searching.  They used to be readily available.

Yours,
David