What mountains are you talking about? How far apart are the fuel stops? What bike/tank capacity/range are you talking? For any trip of that duration through areas with potential drastic weather changes, here is my list in no particular order - this is in addition to standard road trip clothes etc . . .
Rain Gear (always have this but put it here to be sure)
Long sleeve wind shirt - or other long sleeve thermal/heat shirt
silk long john pants
Cold weather gloves
balaclava (in case it gets cold, and if it rains, it helps the rain not hurt the face so much when it pelts you)
Chaps (If I expect it to get real cold (below 40))
SPOT Tracker - Provides SOS capability in areas with no cell coverage
I generally pack ride specific stuff first - all the stuff listed for this particular ride . . . then I pack my clothes, etc . . . sometimes depending on duration and location, you may need to adjust your stinkability meter to make sure you have what you MIGHT need in place of a fresh set of clothes each day.
I know guys who strap gas cans on their luggage racks anytime they ride long distance . . . I only needed one once, when riding through Kansas on my Sabre which had a 130 mile range . . . I have never (knock on wood) had to push my bike or call for gas - though on my last trip my Voyager took on 5.18 gallons at one stop - that's the closest I have got to pushing her . . .
2011 Voyager ABS
Thunder Air Kit
Cobra Tri Ovals/PowerPro
SageBrush Designs Tank Bib
Kury TourTech Mounts
Chrome Psgr board bottoms
14" Rifle Windshield
Russell Day Long seat w/backrest
Kury T/S mirrors
Avon grips w/Throttle Magic by RacnRay
Kury Phase 6 LED Headlight & spotlights
Ride Bells by Sabrehood brothers MinatourAce and Outlaw
Blue Knights VA II
Ride With Pride