Sounds fantastic but I would guess you need planning permission so first call would be to your local council to see if plan is feasible. If you rent out the bus then presumably there needs to be toilet/shower facilities to certain standards, liability insurance, all that sort of stuff.
I notice in Cornwall there are part time campsites that I think landowners can open for 28 days pa. So something along those lines perhaps?
sounds wonderful. I guess how much you could charge would depend on the facilities you install and how you do the conversion. Basic Yurts can be £40 per night in some places. If you did a top notch conversion I don't see why you couldn't charge up to B&B prices. You could offer extras like a breakfast basket including local produce like eggs, milk etc. or a selection of wine, cheese, bread etc. as a supper basket. This might be appreciated by walkers/hikers who travel light. good luck.
------------- Sometimes You're the windscreen; sometimes the fly
As pointed out. If you want to do this you must have planning permission. Once you have that then if you want paying guests you need public liability insurance, then you are covered if you are sued for something.
If you intend to take paying guests then you are a 'professional campsite'. Provided you get the ok from council then you can proceed as intended.
If it's only for occasional guests then council might ok it under the '28 day rule' for temporary campsites. If you get the go ahead then you can proceed as you planned. You could hardly take paying guests without toilet/shower facilities anyway & I can't see liability insurance being that expensive.