Slides aren't such a big issue assuming you can get them out once. After all you will not be operating them after the RV is installed on your property. I'd think about putting bracing underneath the slide.
Also you will want to install insulated skirting, people use aluminum covered foam board as a cheap option. The screen room is a great idea and I'd look at storage sheds, either buying one or building it.
Fifth wheels are much roomier inside. And have much larger basements. No reason to buy new get a used model in good condition with good working appliances.