Greetings & Welcome!
People do box truck conversions occasionally, but shuttle buses or school buses are much more popular because they already have windows. Living in a box without windows is a major failure point for this lifestyle. Windows for the view, the daylight, and the ventilation are your best friends. They help immensely with heating and cooling too.
Building your own is also frequently more expensive than buying an older factory camper that is move in ready. Older, pre-computerized ones cheaper and easier to fix, and much more reliable as well. You should never lose money on an older, pre-computerized, fully depreciated camper.
Building your own or doing a remodel is usually a poor choice. Buy one you like, and use it like it was intended to be used. Ignore people telling you to upgrade to solar panels or anything else, other than maybe a generator if it doesn’t already come with one.
"Proper Planning is preferable to premature failure" ~ DreamLife