we were new this past year, but gained a ton of experience this summer! few things:
I think if you're used to tent camping you are well aware of the space you need/want. one thing we did before buying was go to an rv show. might not be able to find one since its out of season now, but it really helped to see a lot of different models and get an idea of what the space feels like. you'll never get a true sense with pictures/brochures. I see you're in Illinois, my wife and I went to a show in Rosemont last February/March. we thought we knew what we wanted, but changed our minds after seeing so many different options.
like others have said, don't blindly trust the salesmen, they look at your car/truck and say "should be no problem" without knowing any of the numbers! it's very good that you're doing this research.
I would say definitely get a wdh, really makes for a more comfortable and generally safer ride.
for us, a new truck was not an option, so we had to fit our jeep specs. I think it's very doable for your explorer to find something that fits your family. good luck with the search and happy camping!