We also have a new 2015 24R. Here are my observations and responses to your questions.
On the Propane/AC issue. I agree with what someone said above. We used to have a larger motorhome and had to run the generator while on the road in the deep south for the kids in back - but that was because they were so far back - quite a distance from the dash AC - and the Ford chassis AC wasn't all that strong. With the Solera, the kids are so much closer to the dash air and the air is so much stronger, that we've never had an issue. Our propane seems to last forever.
Size - we are a family of 4. Our kids are almost 13 and 10. They both sleep in the overhead bunk - and love it. My husband and I love the slide out queen in the back. I've even napped on it crosswise, when it has been slid in. We also camp for the outdoor experience and tend to only spend time in the RV when we're sleeping or getting ready in the mornings. For meals, usually my husband and one child will cook part of the meals outside while I cook inside with the other. If we do eat inside, all 4 of us fit at the dinette easily. (We love that there is no support post under the middle of the table like our old one - we always fought for foot space). We all take showers in the RV when we're not in a campground with a shower house - with no issues. One tip: we are going to put a fold down door stop on the living side of the bathroom door so that we can flip it down when it's open 90 degrees. That way, all of the vanity and bathroom floor space can be used by someone changing with almost total privacy - without having to fight with the finicky accordion divider (have thought about installing a curtain there instead because we hate the accordion so much).
Storage - we find the outside storage to be exceptional. We travel to softball tournaments nearly every weekend and can fit plenty of chairs, canopies, gear, etc. For inside storage, the only shortage we've found are drawers for clothes. We don't hang our clothes (T shirts, jeans, shorts, etc.) so we needed more drawers. To solve this, I went and bought 2 hanging closet "sweater organizers." It's a hanging soft-sided vertical set of shelves that we can put folded items and towels.
As our final tip - my husband says to watch your head when the rear slide is out. About 3 times now, he's been working getting stuff out of the storage compartments and walked around the back of the RV and hit his head on the bottom of the bedroom slide. The last time he was actually unconscious for a bit. We've decided to place "obstacles" and tie caution tape to the bottom of the slide.
And - make sure if you get a bike rack, that it's one that can fold out of the way when fully loaded. That will enable you to simply fold the rack out of the way if you need to put the slide out for a brief stop over.
The Nutter Family
2015 Solera 24R