If your trailer is not overloaded and the date stamped on the tires is o.k. then I wouldn't worry about tires.
Propane - many campgrounds fill - even if you're not staying there. Large service stations or truck stops usually have a place to fill.
Most likely you can purchase the Pass at your first stop needing it. Most parks sell it. If it's convenient to buy beforehand, fine.
You had better give us all a TRIP REPORT when you return. Many of us having been traveling along with you with our suggestions. :) We'd like to know what did and didn't work for you.
