Packing Dilemmas

How does one pack for the following?

A 3 week trip (actually 24 days)...
(with)
little to no access to laundry facilities...
(and)
a seven passenger van with five seats taken...
(when)
a weather forecast predicts
(the)
high temperature in Las Vegas on Saturday
(will be)
104
(and)
(the)
low temperature for West Yellowstone on Saturday
(will be)
33


(hmmmm...)