So far i have not had to use a tank in my van ,you are definatley limited to how much work you can do on one tank of water and may not be enough like BDCS say just over 1 hours worth of water, that aint gonna get a lot of washing done.
Most people have a outside tap and dont mind you using it ,the few that dont then you have to use a internal tap which i'm wary of especially if its a mixer tap ,those mixer tap hoselock things arent the best and sometimes spray water everywhere if not careful.