You aren't getting responses because you ask too many questions in the same post, and you can find answers to several of these questions by using the search function. There are a lot of answers here. It's best to search first.
Gasoline vs. Diesel, lots of discussion on this, but a lot depends on how much you use your RV. Low to moderate use favors the gas models. Heavy use favors the diesel. Diesels use a LOT of oil and expensive filters at oil change time.
Many campgrounds have a 25 ft. limit on RVs because their parking spaces are too short. You can usually fudge a foot or two.
Slideouts are a good idea. Like anything mechanical or electrical or hydraulic, they sometimes mis-functjon. Also, pay attention to the "closed position" access from inside. If you can't access what you need from the inside when slide-outs are in, maybe you should consider another rig.
Good luck!