Either.
But I would recommend a static system and transfer when you want it. You can also process water at home/base when you are out at work.
Size of r/o depends on how much water you use a day and if you are on a water meter. Once they 'settle' down most cleaners seem to use about 400 litres a day. A 300GPD should suffice for those daily needs with an IBC tank. It should also still produce enough during the winter when production rates drop with input water getting colder.
If you have an r/o in the van then most suppliers fit 4040 r/o which produce water much faster, but are also much more expensive.
A good quality r/o should get your tap/input water of 160 down to around 3 to 4 ppm at 98% efficiency. The remaining tds will be polished off by running the water through a di vessel filled with resin.
Theoretically, you could still clean without the final resin polish, but we won't let the water get above 2ppm before we change the resin.
We process into a 1000l IBC tank and have the di vessel after r/o and before the tank. Water is stored at zero tds. Some windies put the di vessel in their vans and polish the water off as they wash windows. We don't as there are 3 of us and it would mean having to have 3 di vessel instead of the one we have now.