Find a floorplan that will work for you and buy a RV that will work with your tow vehicle. When I was younger, we went across the country in a 26 foot bunkhouse with no slides. We did just fine and had 5 of us and the family dog.
Our retirement plan is a smaller 5th wheel and diesel truck. Smaller because we want to be able to get into state/national parks. 5th wheel because they are more prevalent in the "Extended living" category and they tow better. Diesel because we plan to travel a lot and they get better mileage and produce more power.