Renting a couple different sizes for a weekend would be my recommendation since you've never RV'd before.
By doing this, you'll get to learn all the systems and what to look for when you begin your search.
By trying out different sizes you'll learn what size RV you would be comfortable in - for those longer trips.
You'll also learn what it's like not towing a vehicle behind you allowing you to do some siteseeing from your campsite. Yes, you could do this by unhooking all your connections - water, electric, sewer but the ease of driving your smaller car trumps using your RV to sitesee.
Many RVers are very happy in smaller units, especially if they mainly do short trips or perhaps a long one once or twice a year. I wouldn't advise that you get a big one at first.
By buying used and using it for a year or two you'll then know if you want to go big. Also, when your wife retires you just might want to get out of those winters and head south. Then a bigger one would be good. However, many spend the winters in a smaller RV so don't rule it out. It just might work for you especially if you like to spend time outdoors and doing things.
Above all, definitely test out every system before buying. Good luck!