Reddit mentions of RoofBag Rooftop Cargo Carrier Bag | Made in USA | 15 cu ft |Standard Waterproof Luggage Car Top Carrier | 1 Yr Warranty | Fits ALL Cars: With Side Rails, Cross Bars or No Rack

I think that you will prefer doing two cars. It will give you more freedom at your stops if people want to do different things. Also, in my mind an RV is going to be a huge hassle, unless you are only camping or staying in the country.

As far as a rooftop carrier, you do not have to have a rack or buy a hardtop. It is cheaper and in some ways more convenient to buy a roof bag. We purchased this one on amazon last year and it worked great. When I bought it the price was $68 - it looks like it went up, so you could shop around for a cheaper one. The benefits of a roof bag are: 1) cheaper, 2) don't need a rack, and 3) you can easily take it off and store it in your vehicle with your luggage if needed during stops (discourage thieves if you are worried). We put ours through hell and it held up great.

For our trip, we only had about 16 and a half days, so two weeks plus the weekend. We had to stay on this schedule due to work/time-off - you could do a lot more than us with an extra week. You can buy any park passes at the park and I would just pay for tolls as you go. Unless there is a specific tour that you want to do which could sell out, most things you don't need to buy before hand.

It sounds like you guys are taking a very similar route to what we did - I have listed it below so you can get a feel what what is possible:

• Day 1: left North Carolina Friday night after work, driving all night.
  
  
• Day 2: Stopped in the morning at the St. Louis, MO, Arch. We only stopped to see the arch for about an hour and then kept driving. Arrived in Denver, CO at the end of the day, driving 28 hours straight if you don't count St. Louis.
  
  
• Day 3: Left early and spent the day at Rocky Mountain National Park, CO. We left around 6PM to drive about 5 hours into Utah and spent the night at a motel.
  
  
• Day 4: Drove all day and arrived in the West side of Yosemite National Park, CA late in the day.
  
  
• Day 5: Spent the day in Yosemite National Park. We all loved Yosemite, probably my favorite national park.
  
  
• Day 6: Left early in the morning and drove three or so hours to Napa Valley and did a vineyard tour. After that we visited Muir Woods (huge trees) and then drove into San Fransisco where we would spend the night.
  
  
• Day 7: Spent the day in San Fransisco. We all love San Fransisco, it is one of my favorite cities - very unique.
  
  
• Day 8: Left San Fransisco early to take the PCH (highway 1) down the coast to Los Angeles. It is about a 6 and 1/2 hour drive, so we made a day of it and stopped at Carmel Mission, Pfeiffer Beach (beautiful with purple sand), and Hearst Castle, then finally arrived at our hotel north of LA.
  
  
• Day 9: Visited Hollywood and Santa Monica. Unfortunately, horrible traffic kept us from doing much more.
  
  
• Day 10: Left our LA hotel early and drove to Las Vegas, arriving in the afternoon. Explored the city, ate at Bacchanal Buffet, and partied.
  
  
• Day 11: Partied in Vegas.
  
  
• Day 12: Drove to Phoenix, AZ.
  
  
• Day 13: Made a day trip from Phoenix to northern AZ. Stopped in Sedona (Red Rock country like Road Runner in the Looney Toons and seriously awesome to see), then visited Walnut Canyon National Monument, and finally the Grand Canyon (which is an obvious must see). Then drove back to Phoenix.
  
  
• Day 14: Drove around 25 hours through the night to Nashville, TN.
  
  
• Day 15: Slept and relaxed, then explored Nashville a bit at night.
  
  
• Day 16: Explored Nashville.
  
  
• Day 17: Drove home.
  
  A few more things:
  
  
• States like Nevada and Utah are VERY empty as far as other people. Do not let your vehicle's gas tank get below half way - in fact, I would fill at 3/4 of a tank. We had a close call because of this.
  
  
• The Rockies and Yosemite have some very steep and narrow roads. This is particularly true if you go north of Yosemite (it is ridiculous) to get around the mountain range. Your vehicles are going to be under serious stress weighted down with 4-5 people and luggage. Bring a pre-deluded antifreeze bottle incase of over-heating. Be cautious about driving through these areas at night (it is pitch black).
  
  
• Weather is different out West. We experienced a flash flood that nearly swept our vehicle away. Another time we had to reroute because of a wild fire.
  
  
• Driving is not as boring as it seems. Despite all the driving, we loved watching the landscape change. Places in Nevada and Utah look unreal compared to NC.
  
  
• The trip was not as draining as it sounds. For the most part we were full energy and ready to go, even in our final days -- when we got home, we all wished we could keep going. As I mentioned before, using minor sleep aids helps with this a lot. Make sure you rest in the car when it is not your turn to drive. Make sure that everyone is required to drive 3 hour shifts. If it is four people, that means you only have to drive 3 hours and then have a 9 hour break period. THAT SAID, I'm not familiar with your group and some people may not cope well in this sort of situation.
  
  
• Sometimes the trip was uncomfortable or even scary (a few close calls), but it made the best memories. I have been to nice resorts, vacations, etc., but this trip is the one we talk about the most.
  
  

That's an interesting trade but unfortunately it's a no go (maybe a yard sale in the past?), this is the one we had though, looks like the rack option is in stock and they now make a smaller one (11 cu ft vs the 15 we had)

https://www.amazon.com/RoofBag-Explorer-Waterproof-Soft-Carrier/dp/B0042H6BA0/ref=sr_1_2?ie=UTF8&qid=1467288683&sr=8-2&keywords=roof+top+bag

Looks like there's some cheaper options, I just know that's the one we had and it was heavy and durable, I'd definitely get or fashion an anti skid mat.

EDIT: On second thought, maybe this was outrs, they all look the sameish:

https://www.rei.com/product/677675/sherpak-go-15-roof-bag