The Shadowruner companion has custom lifestyle rules, but unlike some of the older ones nothing about living in a vehicle (yacht, RV, etc)

Or am I missing something?

Just run it as a normal lifestyle cost. Even if the character is living in an RV instead of a house, he still has all the normal costs. Even though he's not paying rent (or a mortgage), he still has maintenance costs for the vehicle and living space, as well as hookup fees for power, water, and sewage. What makes an RV great is that it can move.

Amenities upgrade in Arsenal covers this, in part, I think.

I would agree with Erik, the costs are different, but probably not much lower or higher. Though with the custom lifestyle rules, maybe limit "Comforts," skip "neighborhood" completely and I'd say make "Security" 0, but based on whatever security systems the player has in their RV. Other than that though, living in an RV probably wouldn't really affect your Necessities or Entertainment too much.

In the AR age, i suspect one can live in a coffin without missing out on much.

Hell, i sometimes wonder about heading to Asia and try out some netcafe living:
http://en.wikipedia.org/wiki/Net_cafe_refugee

LOL, Miami Vice. Don't forget the pet gator.