A few things that I would consider when installing an under van tank:

-leaning out of the sliding door to open/close the propane tank can be very
convenient

-while you want the tank to be high up and out of the way most fillers will
need to stick straight out from the fill connection.  Keep this in mind and
make sure that there is room for the fill device (whatever it is called).
Rotating the outer edge of the tank down or simply lowering the entire tank
are two options if the tank is too high.  My second tank was always causing
problems at fill stations so I installed spacers on the outward end of the
tank mounts that are about .5 " thick and this solved the problem.

-wherever you install the tank route the lines in the most protected way
possible, minimize flexing by securing the lines to non-moving anchor points
and pad/protect the lines as needed.  I used heavy zip ties, foam blocks and
the thick gray plastic sleeving that I picked up from bus depot.  I guess
this was originally specified for electrical or gas line sleeving/shielding
but it seemed appropriate for chafe protection and for protection from mild
debris dingage in the slightly exposed undercarriage locations that the pipe
crosses through.

-A bit of extra pipe/line length never hurts and can help avoid crimping and
problems that may come as you need to move things around a bit during
install or future servicing.

-if you plan to use the propane a lot two tanks is great for so many reasons
and well worth the hassle/expense of install in my opinion.  An extender
that allows for external bbq tank integration at times would surely provide
many of the same benefits.

Enjoy the propane system,,

Jonathan Poole