1) more space (especially as I'm a 1.87m guy)

2) proper toilet and shower instead of chemo toilet and something that's more a droplet distributor than a shower; in addition you can physically separate the toilet and its gases from your living room.

3) trucks and buses are designed for maintenance by the fleet owners and have space to actually work on the components, in contrast to loads of car based RV models

4) RVs at truck/bus scale are either expensive or so worn-out that a rebuild is the same cost

5) customizing - I like building stuff, for one, and normal RVs don't come with adequate 230V outlets or other IT infrastructure.