This seems like an excellent idea to me, since the structural components are already in place. I am not sure about the longevity of utility trailers and how resiliently leakproof they prove to be. But for saving time and for cost effectiveness (and especially for those with less building experience who will be doing it themselves) this seems like a really great avenue to stroll down.
Cargo trailer conversions:
Cargo trailer campers:
One unit for sale with great design implementations. I love the shower/washtub:
Camper conversion for $2k:
Insulation on these campers?
Structural integrity when adding windows and doors?