I have no experience in this but it occurs to me that the van won't be inspected to the same standard as a building permit inspector or some such.
Imagine you screw up the ventilation and have a carbon monoxide problem, or, you screw up something electrical and put yourself at risk of fire. I suppose there are a lot of spotty contractors out there that still manage to do their thing on more traditional dwellings too, but DIY in the tight space of a van seems like some of the things described are not without risk.
There was a time when everyone managed to get by without building codes. Sure we had a few fires and collapses, but statistically speaking they were still outliers.
Don't let the 1% risk of bad hold you back from trying good, is my only point.
Imagine you screw up the ventilation and have a carbon monoxide problem, or, you screw up something electrical and put yourself at risk of fire. I suppose there are a lot of spotty contractors out there that still manage to do their thing on more traditional dwellings too, but DIY in the tight space of a van seems like some of the things described are not without risk.