If you're selling them as a company, you must abide to the DOT regulations on campers/RVs/Trailers, and build it to code, just like a house.. Those who build them as a DIY/hobby, and sell with a AS IS title I believe are in a sense exempt from this (please correct me if I'm wrong)..
I was made aware of this when building my ambo camper, although it was a DIY build I was informed of several regulations and codes that I was breaking, I then changed the design so it was built to code- allowing it to be registered as an RV. Biggest one was propane storage, and stove top distance to overhead objects. The member Patoz is very knowledgeable about this topic, and I'd suggest reaching out to him.