Richard

<I have the idea of a second propane tank, and interconnecting
the low-pressure lines (after the regulators.)  Then, I could
run one tank empty, turn it off, turn on the other tank and
thus alternate tanks.>

Yes, I have done this and had no problems. Although I see no harm in having both
tanks turned on at the same time, I have only used my second (backup) tank when
the main (built in) tank ran empty and turned off.

<Question:  will this work?  Can the "off" tank have pressure
on its regulator on the "back" side?  That is, will the regulator
be OK or will it leak propane in that situation?>

If both regulators are rated the same, then the pressure after the regulators
should be the same whether the second tank is connected or not or turned on or
not.

Main Tank     -> On/Off Valve -> Regulator -> T Coupling -> Van Range and Fridge

Backup Tank -> On/Off Valve -> Regulator ----^

-Larry