Haha, but what are you hauling? What size motor? Weight of truck and size? What route, highway? City?
All of that plays a factor into gas mileage. Obviously the more highway, less weight and "smaller" size of engine make for higher gas mileage out of any vehicle; not just diesel.
And there is no way in hell you're going spend $3000 a week in diesel unless all you do is drive from coast to coast and fill up in big cities. I doubt you'd spend anywhere close to $3000 a month if you drive conservatively.
I think this is a great idea. I have been planning to do the same thing with an older 7.3 Ford but put a bed camper in the back, or make one if time and money permit.
Here are a couple of suggestions for you.
1. Look into converting the bus to run off natural gas. It's a fairly simple, straight forward method of converting. You'll have a cleaner, cheaper fuel and you'll average around the same gas mileage as regular diesel. To convert you need to change the injectors, lines and the fuel tank; as well as a couple other things. This can run up to about $4000 so it's something to look into later on down the road. For now I'd run it off diesel until you get bored and want a new project or have some extra cash laying around to dump into the bus.
The only down side is finding places to fill up with natural gas. Not many gas stations have it on the pump. Usually bigger cities; such as Denver, will have 9-10 natural gas pumps. If you plan your trips and fill up before you leave you should be fine within the range.
2. WINTERIZE! Make sure you do this. Buses are cold as fuck and if you're using it during the winter to ski you're going to want to block off a majority of windows and insulate as much as you can to keep the heat in. That will also help to cut back on the heat you'll have to use from another source like a stove. Look up some instructions online about winterizing RVs. It will give you good info on making sure you don't have any issues when the temperature drops.
3. Don't take the short route hooking everything up. If you're going to do it, do it right. Hook up the plumbing, heating, electric, etc. the right way or else you'll end up having headaches later on down the road, same goes for the winterizing.
Other than that have fun with it. Making you're own camper out of an old school bus is a lot of fun. Gives you free range to set it up however you want!
Also one last thing. Say you want to hook the bus up with electricity; go to Harbor Freight. Usually they'll sell solar panels for like $250 for the whole kit. Slap it on, bolt it down and you're good to run some lights and maybe a microwave.
has a thread on RVs. Check it out for some ideas, 54 pages of pictures.