Is my truck supposed to leak down air?

Probably one or many of the hundred air connections on the truck. Get some soapy water in a spray bottle, and start spraying every connection. Fix anything that bubbles. For every air leak you can hear, you might have ten that you can't.

 

If you have a Volvo, most likely one or more connection at the firewall and possible throughout the entire truck. They use push-in connectors which easily leak.

 

Yeah, not normal. Truck should hold air all weekend. Many places to look. If any your bags are empty after then I'd start there. Possible leveling valve or bag leak. Fan hub is another place. Then most air connection that are push in type.

 

Not normal at all. Look at the quick connect fittings at air bags.

 

Tractor alone you're typically allowed what, 2 PSI max per minute of air loss?

That said, most trucks I've seen hold at least 60 PSI when parked overnight.