Hi and Welcome. I take it from your itinerary that you are amusement park junkies??? I offer the following comments for your consideration. Our family has traveled cross country at least 4 times in a car, and twice in our 27 foot RV. This has formed the basis for this response. First to your questions.
1. One AC should be fine
2. A shorter RV will be lighter, thus allowing you to carry more weight (people and stuff. A shorter RV will be easier to move around in gas stations with less tail end swing, easier to park at stores, restaurants, and campgrounds. A shorter RV is easier to drive in somewhat congested areas like cities, and some small towns like Jackson, WY.
3. Tail swing is more likely the issue. You may be used to it given your driving experience with larger vehicles, but it can be a problem for your wife
4. Slideouts are a personal choice. You can't have them out while driving LOL, and you are driving a bunch. If you do chose slideouts, make sure you can access necessary items when they are closed-in, and remember that slideouts add weight which subtracts from the amount of people and stuff you can carry.
5. Genset fuel consumption is negligible.
6. You hopefully won't have to sit in repair shops much on your trip, but frankly it depends on the quality of the unit you buy and how it was maintained.
Comments: To each their own. But, you are planning to travel through and in some of the most beautiful and historic areas in the country. Do you really want to spent that much time in amusement parks? In San Antonio, I suggest adding the Alamo to your list, and the River Walk. There are a also a number of historic Missions there. You can "do" Great Sand Dunes in a relative short amount of time, then consider driving west to Durango and take a one-way train ride on the Durango and Silverton narrow guage RR. Take the bus to return, saves time. I think the Grand Canyon should take a couple of days to view from different points. From there to Anaheim is a full-day drive. Disneyland will require at least two days. I would spend more time in San Fran that you have planned, but it's your plan. As I remember, it will take 4-6 hours to get to Yosemite from SF, and I think there is only one or two routes into the Park for RVs. A visit in the park can take 2 or more days. If you plan on staying in SLC, consider the SL KOA. It is right on the light-rail line between the Airport and Downtown, easy to come and go. Gran Teton can take a couple of days at least, one for multiple views of the mts., part of one to tube down the Snake River, one to boat across the lake and hike to the hidden falls, and more. Yellowstone is bigger than a couple of New England states combined. If you want to see a good part of that magical place, you will need several days. You can spend a fair amount of driving place to place. In South Dakota, I would suggest you consider the Crazy Horse sculpture in addition to Rushmore (not too far), and just east of Rapid City is the Badlands National Park which is well worth the visit. Avoid Chicago area to any degree you can. You can spend hours tied up in traffic. Ohio should be a straight shot, but take the ring-road around Columbus. Once in PA if you are taking the turnpike, be careful from Donegal to Carlisle exits. Very windy with steep grades over the mountains. Also, keep in mind that there are four tunnels along the turnpike in this stretch, with two back to back. Hershey Park is a nice family amusement park, but it is not Disney! It will take you about 18 hours driving time on very busy I95 to WDW from NJ. Be especially alert as you travel through Phili, Baltimore, and the DC metro areas. Avoid rush hour in all three cities to the degree you can. DC area is always bad, avoid if you can. WDW will take four days to see the four parks unless you are already familiar.
Best wishes on a safe and sane trip.
