One danger with these things is if you're not careful, the warning flag becomes a normal part of the cockpit environment while you're parked, and you cease to notice it. This phenomenon resulted in an airliner trying to take off with the parking brake set, several years back, in spite of a bright red flag near the pilot's line of vision. They're a good idea, but don't rely on them too heavily. They're no substitute for a good checklist, carefully followed.
I'm still working on mine. So far I've got:
BEFORE STARTING ENGINE:
Utilities - DISCONNECTED
Propane valve - OFF
Furnace thermostat - OFF
Cabinets - LATCHED
Penthouse top - DOWN & LOCKED
CHECK BLIND SPOTS FOR OBSTACLES - GOAL*
AFTER STARTING ENGINE:
Oil pressure - NORMAL
Ammeter - CHARGING
House battery - 13.8 to 15V
Parking brake - RELEASE
*GOAL is an expression I learned from a trucker -- it stands for Get Out And Look
I've had thoughts about interlocks but nothing firm yet...a previous owner put tape on my latch hooks to stop them from squeaking so I'd have to undo that before using them for any kind of electrical connection.
Actually, one pretty simple idea I had was to put the ignition key on a ring I could attach to one of the front latches. That way I'd have to touch the latch and think about it before I could start the van.
A relay interlock to prevent starting the engine with shore power connected is another thought -- I have a plug-in hybrid that won't let me shift out of park if the charge cable is connected.