rental cars

11 days SUV:
kayak.com 9/16 - 9/27, JAC - BIL $1027.55 for a Jeep Grand Cherokee or similar from Alamo.
7 days full size car:
kayak.com 9/20 - 9/27, JAC - BIL $377.89 for a Generic Full Size car from Alamo.
11 days full size car
kayak.com 9/16 - 9/27, JAC - BIL $680.20 for a Generic Full Size car from Alamo.
4 days full size car (will probably need 2)
carrentals.com 9/16 - 9/20 JAC - JAC $128.01 for a Toyota Camry or similar from dollar.
4 days van:
orbitz.com 9/16 - 9/20 JAC - JAC $307.72 for a midsized van Caravan or similar from Dollar.
UPDATE as of 8/21/14, found a midSized SUV directly from Dollar for $179.90.

For some reason renting one car for 11 days straight costs more than renting one car for 4 days and then a different car for the next 7 days as shown below:
1) Renting a car for 11 days = $680.20

2) Renting two cars for the 1st four days, and then two different cars for the last 7 days costs
($128.01 + $377.89)  = $505.90, a savings of $135.

The van is only $51 more than renting two full sized cars for the 4 days and will simplify parking and allow you all to visit and plan on the way to Yellowstone.

So, my recommendation is each family pays $377.89 for a full size car for the week of 9/20- 9/27, and each family pays $103 for 1/3 of the Grand Caravan for 4 days?

A complication would be getting people and gear back to the condo on 9/27.  Lynn's flight changed such that she is arriving around 7p.  My flight changed also, but I have not accepted the changes.  R&T could drop Jim and Casey off at the condo on their way to the airport to exchange to a smaller vehicle.  Update, Connie's flight arrives at 8p and Casey is going to the airport to rent a car for the Phelpses so it will be done ahead of time.  It is 10 miles/16 minutes from the airport to the condo.