Without a few more details/guidelines it's hard to come up with any specifics, though some of the ideas on this thread might be applicable to your potential situation.
http://www.bushcraftuk.com/community/showthread.php?t=13046
What sort of size van/bus are you thinking, small van, minibus, 7.5ton, double decker?
If you think your going to have a flame inside your probably best getting a deisel and not having to worry about possible explosive vapours building up.
An idea of budget ( ontop of the van/bus costs ) might help too, ball park figures like £50 or £500 would give an idea, also how handy you are at things, woodwork/metalwork/electrics/plumbing, would all help people tailor their advice to your situation.
If your going to be on private land and the owners don't mind a few people pitching tents for a weekend you might find that some of the more skilled/helpful people on the forum might be willing to pop along for a day weekend and help you with some of the things you find dificult.
Think about anything you might already have that could be reused, eg carpets for lining floor, walls ceiling, heavy curtains for doors, windows or partitioning into smaller spaces eg a sleep space. 2 layers of heavy curtains an inch or 2 apart that can be drawn back out the way can make an awful lot of difference on cold windy nights while taking up little room.
polysterene tiles ( not near flames or sparks ) any tarps, sleeping bags, mats would of course prove usefull as might keeping a chair or 2.
If your going to have the floor space, a 2 person pop up tent on a layer of wood and a couple of carpet ( or even cardboard ) can make quite a cosy inner sleeping chamber in winter and easily stowed when not in use.
A few ideas, that are worth thinking about even if you've a reasonable amount in the budget ) but certainly if you've very little.
Water : Carrying 2 ( or even 4 ) 2 litre plastic drinks bottles around, to be filled at any opportunity, can make quite an impact on your fresh water supply for cooking, drinking, basic hygenie and washing up, even if you have a 100lt tank onboard.
If you've got daily access to alternative fresh water heating ( eg work ) a 1 or 2 1lt themos flasks are worth thinking about, topping up with hot water just before hometime and keeping any hot water you've heated and not used.
Toilet : Use pulic ones whenever possible, keeping a dedicated sealable 1 or 2 gallon unrine container onboard and emptying it into public toilets can make things a lot easier, especially on long dark cold weekends, not only does it save your tiolet facilites but it saves loosing any built up heat inside by opening doors going in and out.
Clothing and cleaning your clothes might take some consideration, even if you have access to someone washing machine every couple of weeks, a few of sets of lightweight quick drying underclothes might well be worth investing in. Same principle with a easy wash/dry layer for the bedding that's next your body. Both these can be easily/discretely washed in any sink with hot water supply and small things like that can be as much about keeping up longer term morale as they are hygeine.
