My question is; In large cities, will we be able to find safe and legal places to park overnight while we sleep in the RV and how?
Large cities are the least likely places to find RV parking, let alone a place to spend the night, even in a small RV such as a Sprinter. So you might, in many cases, have to park 20-50 miles out of town to spend the night, then drive in and see what you wanted. There is an app for your tablet/phone called Allstays
Camp and RV which can help you find campgrounds, truck stops, even some Walmart parking lots (not all), and more that will serve as places to spend the night.
It would actually make more sense to buy in Florida, where you live, simplifying vehicle registration, etc. then start your travels from there, rather than the hassle of getting one clear across the country. It's not like renting a car.
As Tom says, it would be wise to take a couple of short (perhaps weekend) trips near home to find out what equipment/supplies you need to get, and to figure out how to operate the various systems (water, sanitary, electrical systems, furnace, slides, shades, to name a few), and to learn the precautions you need to take to minimize the chances of problems. You'll want to take time to study the manuals that come with the rig, too. It's not intuitive.
We'd hate to see you with problems that would hinder your enjoyment of RV traveling, so you might also want to peruse the Library here (button above) to find checklist and all sorts of pertinent information about RVs, even buying and selling them.