When we purchased our used Interstate, installing an interior propane switch was one of the first upgrades we made. I can't imagine why Airstream continues to avoid addressing this issue. AIRSTREAM - ARE YOU LISTENING!!!!????
There are several discussions that cover the installation of an interior switch for the propane solenoid. Some of them more helpful than others. I don't think any of them refer specifically to a 19' Interstate. Study the three threads shown below - in particular, look at post #12 in the third thread. I used the PDF file referenced there, as a guide, to add an interior switch for our propane solenoid. Our layout was very similar to that shown in the PDF file although there were several differences in our layout. You will need to adapt to your 19'.
I basically went to the outside switch and added additional wires in series. Routed the wires to the inside and put the new switch in the same panel as the other electronic monitors. Both switches must be on for the propane to flow. I simply leave the outside switch on all the time and turn the propane on/off from the inside. The interior switch has an LED to remind me when the solenoid is on. The exterior switch remains useful when refilling the tank.