Everyone with experience says that.
Honestly, I have more storage already than I actually use in my house. I'm still trying to imagine what I'm going to put in four chester drawers (I don't even use two at home). Hanging closet consist of a couple jackets and a dozen t-shirts. A place for electronics (drones, cameras, etc). A place for ammo and guns. Same for the kitchen stuff (I've never needed more than one knife, fork, or spoon to cook and eat a meal). One skillet, one pan, one pot. A coupe plates and bowls. Can opener, bottle opener. Pantry for canned and dry goods. Little fridge. The little microwave I have is the same tiny one I use at home. I never use my oven at the house since I got a little InstaPot airfryer four years ago (so I got another one for the truck). It will only ever be me and the dog traveling. I'm one of those people who could live out of a backpack and a duffle bag (toured most of Texas and even Scotland on a Harley that way).
Now if I were traveling with a gf or wife (or godforbid, kids) then I could see wanting every square inch of useable space dedicated to storage and still getting bitched at for not having enough. But me and the dog don't require much. I'm far more short on external space for things like tools, fishing gear, recovery gear, etc. Outside I'm going to need way more storage than I can fit (I have the M1082 for beach camping but will likely come up with a better solution for actual travels)
I'd rather keep an open feel to the space inside, to be honest. If after some time I deem more is needed, cabinets are easy to add.