An r/o system on its own will rarely take the water down to zero, so usually they are used in conjunction with di. This can be either as part of the r/o system, or separate. If it is separate, it can be put in the van, which is what i do. You can get away with di only, which would typically be used in the van, but it is only cost effective if your tap water has a low ppm reading, i.e, below about 75ppm. Even then though, if you can use an r/o system, it will work out cheaper to run long term.