During the past twenty years I made several trips to the Rocky Mountains, the Pacific Northwest and California. Most of the camping trips were to the "Rocky Mountains" area, specifically South Dakota, Montana, Wyoming, Colorado, and Utah (and Idaho). I have also traveled around Oregon, Washington and British Columbia some but most of that was not camping. Flying to a Rocky Mountain city (usually Salt Lake City or Denver) saved at least 3 days, each way, of boring driving from the Mid-Atlantic region. We would target a start/end launch city for a "loop trip" due to the cost of drop-off fees with rental cars. That worked out fine. Our trips were usually 2 weeks in duration.

Looking at your desired destinations, all great, really adds up to 4 separate loop trips: Northern Rocky Mountain states; the Great Southwest; California; and the Pacific Northwest. It would be a stretch to justly do two of those "loops" in a 3-week trip driving from Philadelphia. Several people have stressed the time/distance calculations of programs such as Google Maps and suggest dividing distance by 55 mph. IMO, that is fine for sprints from Philly to Denver, but then it is game over if you are interested in camping or sightseeing. For the most part, great portions of those regions favor traveling along the US highways, not the interstates (it is not like driving from PHL to Orlando on US 1 or US 15). For example, from Denver to Black Canyon of the Gunnison (camping!) to Durango on your way to Mesa Verde. Or Denver to the west slope of Rocky Mountain NP (camping) to Gunnison (campout) to Mesa Verde.

If you were to fly, then Southwest Airlines is probably a good choice with their 2 free bags per person plus a personal item (e.g., a backpack). SWA also tends to be more supportive in your baggage situation then other airlines these days. You need to keep your bags within a 62 inch total (Length+Width+Depth) and under 50 lbs. Bag #1: Cooler packed with camp stove (propane powered, but no propane cannisters whatsoever), utensils, table spread, plates, cooking pans, skewers, etc.). Bag #2: duffel bag with tent, sleeping bags ground cover (wrap hatchet/ax in a blanket). Bag #3: Clothing. Bag #4: More clothing and/or camp supplies.

One excursion plan that worked well for us was flying into Salt Lake City and then driving to Pocatello for camping out provisioning (fresh food, dry ice, matches, propane for camp stove, etc.) and a night in a hotel. Early in the morning we'd drive to the West Entrance to Yellowstone to secure a campsite for the next few days. From Yellowstone to Glacier NP, a beautiful route is via the NE gate to Red Lodge and on to Missoula for a provisioning night-over on the way to Glacier NP; or, to Custer State Park (camping) for the Badlands, Wind Cave area. The Yellowstone to Red Lodge route is one not to miss... tons of switch back, snow and ice at the pass and allow plenty of time. Scenic and slow :) Anyway, digressing... we would usually camp for a few nights, then break camp early and drive to a gateway town for provisioning, touristing and sleepover and then get an early start for obtaining a campsite at the next destination. We provisioned in Missoula for driving up to Glacier NP.

For the Southwest: Denver, SLC, Albuquerque and Phoenix are all good loop cities -- whichever has reasonable rates non-stop and affordable rental cars (airport cities can vary due to competition and fee structures + extra taxes). Speaking of rental cars, a larger sedan is much easier than a smaller sedan if you are camping; a station wagon style vehicle is easier than a sedan; and a minivan is easier than all of the above!

Have fun... and dig into your AAA USA and regional maps with pins and highlighters. There is more to do than you can imagine in any of the "four circles" I mentioned above.