Welcome Ched, I'd say convenience of access for filling the van long term would be my deciding factor. If the garage is on your property getting a water and electricity supply should probably not be too difficult. Neither should the waste water drain. I'd guess you probably have drainage for gutter downpipes you can feed a pipe into. You could build a shed around an Ibc and processing equipment, if you wanted to and if you have the room.Cheers @Davy G and @Scots lad.
My first job after my apprenticeship was site electrician but I never sat the regs. So I know how it's supposed to be done and could do most of the work but for connections and cert I would get a qualified sparky.
I'm trying to decide on location. We live on a corner so could get electricity to garage at rear then I would need to get water and a drain there as well. Alternatively I can park at side of house next to tap and drain so just need power to process.