My wife and I traveled thousands and thousands of miles with our first travel trailer. It was an 18 foot Dutchmen Sport, measured 21 feet long, bumper to hitch. We initially towed it with a Chevy 1500 Suburban. Put lots of miles on that puppy, until the transmission blew. Then we moved to our first dualy (used). It was a Chevy Silverado gas 3500, a marvelous beast, except we didn't realize the frame was rusting too bad when our repair shop advised we never tow with it again... We traded for a diesel dualy (used).
Meanwhile, we traveled extensively in an 18 foot (box size) travel trailer and did just fine. Size is not the important thing. What is important is how well you can adapt to the size.
My parents had an 18 foot 1963 Phoenix travel trailer, purchased new in 1962. I was 7 years old. And my parents traveled in that little trailer with a family of 3 kids and a dog all over the country, from Michigan, to Texas, to Florida, to New York, and everyone had a marvelous time. My dad was self-employed and set his own work schedule. It was common for us to travel a month at a time. I'm pretty sure that's what gave me the love for RVing.
By the way ... no matter what you get, get weight distribution and sway control. You may think you never need it, but it only takes one time to lose control and your family members end up in a morgue. Think about that. When RVing ... the safety of your vehicle is the most important the most primary thing. Safety with your vehicle supersedes everything else.
My parents "rig". I'm the runt, balled headed kid squatting like a contorted lobster on the sand.