lol you guys in the posts above can't even offer advice yourself?

It all depends on how much water you go through a day.
I presume you are talking about a static tank yes? Depending on what your water need is, either a 300 or 450GPD, or perhaps a 4040. They differ on how much water they make.
Apart from that, you need a transfer pump, transfer hose from static tank to vehicle tank, outside tap, drain, and power supply if your mains pressure is too low and you need a boosterpump for the RO.