Welcome to ecomodder.
They're already pretty smooth as RVs go.
https://www.google.com/search?q=ultr...w=1411&bih=625
What's the underside like ?
On some pics, it looks like they already have some sort of belly pan to smooth out the underside - though it could be improved a bit.
Does your UltraVan have lots of stuff hanging on the sides or on the roof ?
However, increased tyre inflation and changing / improving the way you drive it would probably be the most effective changes.