here4fun183,
Those slide-in campers for pick-up trucks are usually suffering from dry rot and worse, they have 1"x2" stapled framing. Prepare to breathe mold if you decide to go with the slide-in camper.
First choice, I'd build a large, tall teardrop on the utility trailer.
Second choice, tearing the pop-up down to the chassis and building on it...but be prepared for having to cut some and weld some on it.
Prem
My goal...
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...is to live in a trailer.