I think the problem with a duffle bag is it's going to squish all our clothes so shirts and trousers will need to be ironed when you plan to use them and we'll be on the road so much that it could be get quite expensive. We also don't own any dufflebags and a decent wheelie one is north of £100 each
I literally landed back on Saturday from a Cali trip including LA, LV, SD etc.... (3 weeks, 2700 miles in a Mustang). Our clothes were perfectly fine for the entire trip. You won't be needing more than 1 pair of trousers really and everything else is just folded into the bags and keeps fine. My wife hung up her expensive 'going out' dresses when we arrived at each place but that was it.
We bought our bags in Debenhams for £40 each, we've now done 5 (I think!?) road trips in them over the last 2.5 years (so roughly 15 weeks of driving round the US in a Mustang) and I've never had ruined clothes.
But there are quite large drives...LA to LV is 4 hours. LV to SF is 14 hours. SF back to LA is 10 hours. Are mustangs really suited to those kind of drives? i.e. ignoring the lack of boot space, are they actually comfortable cruisers?
Yes, for the places you are going you really do need a Mustang. It's not the best car, nor is it the most comfortable but it's perfectly fine. The rental spec ones have heated and cooled seats, climate, auto, leather, cruise, USB, bluetooth and you can easily relax in one. I often just chucked the cruise on 70mph, roof down, seat cooler on, plugged in the iPad and drove enjoying the amazing views that part of the world has to offer.
You'll really appreciate the ability to remove the roof as you drive down the PCH, down the LV strip, over the Golden Gate bridge etc....
I did a few long days in ours last week (North rim of the Grand Canyon to Monument Valley, and then to Flagstaff - so roughly 7 hours of driving) with the roof down the entire time and it was fine.
LV to SF is 14 hours but surely you are stopping in Yosemite so that will be broken up?
p.s. Is the Camaro/Mustang same category? any thoughts about the Chrysler 200?
Alamo / National won't have any Chrysler 200's (I've not seen a single one in the lot in my trips). Mustang or similar means either a Mustang or a Camaro. Usually I arrive into LAX and they have a few Camaro's and maybe 1 Mustang, I had to wait about 5 minutes the first time I went as I insisted on a Mustang. The Camaro boot is smaller than the Mustang and the opening is smaller so it's harder to get bags in.
Book a Mustang or similar, wait the extra few minutes if required to ensure you get one and you will really enjoy the trip and appreciate that the Mustang is the right car for this drive.