We began full-timing in 2014 in a 43' 5th Wheel Toy Hauler. Based on over a year of perusing FT forums and asking questions, we decided to get rid of virtually everything we could not take with us. We had the kids come get what they wanted, gave family heirlooms to other family members, then had an estate sale and sold the rest. We did save a few tubs of items we knew we wanted to keep and those are stored in my wife's sisters attic. It is primarily family memorabilia. I digitized all of our home movies, pictures and photo albums and gave the hardcopies to the kids. I built a tool collection of ones I felt I'd need (to which I've added over the years) and the wife did the same with her craft stuff. All in all it has worked out well.
We were both all in on the new lifestyle and downsizing from 3000sf to 450sf has been no issue. If you are going to use a storage building, make sure to not let it go too long. You'll find most of it was just "stuff" and you'll be paying to store things you really are not invested in. We have zero regrets and our first 7 years has been wonderful. Good Luck.