When was the last time on a trip that you got two flats, of which at least one of them was not field repairable (due to any reason) ?
How long does a full bottle of propane last you between refills?
My answers to those questions are:
Never. A spare for each seems more than reasonable enough for U.S. trips. Heck, if the furthest that you can get away from a McDonald's is 145 miles by road, there's bound to be a tire shop within that range as well.
If you are thinking OZ Outback, Africa, or South America, then maybe a third is appropriate. I would make sure to have a good tire 'boot' for sidewall patches (purely limp out mode), and a tube of the right size vacuum sealed in a foiled mylar computer parts bag goes a long ways towards not needing another spare. That buggy in my avatar had a tube in a tubeless left rear tire when that picture was taken.
Don't know for sure, on the plus side of 6 days somewhere. 6 days of cooking breakfasts and dinners for 6-10 guys is the most that I've used a 5 gallon propane tank, & I've always come home with fuel in the tank. Based on this I plan to eliminate one of the two tanks on the TrailBlazer trailer.
I used to swerve around my hallucinations, now I drive right through them.