The main question you need to decide is 'are you going to process your water at home into a static tank and then into your vehicle tank or will you want to process it all in your vehicle?'
If you want to process at home into a static tank then the 300gpd will be the most economical (set-up and maintenance) way to achieve this. It will produce on reasonable pressure about 700-900 litres per 24hr day. More than enough for your needs, especially if you have a 1000 litre static holding tank you will always have some in reserve.
If you need to process in your vehicle, then as long as your are on good water pressure you could just manage on a 300gpd in the vehicle processing overnight (12 hrs = about 400 litres). However the '4040' type system would guarantee a delivery of about 100-200 litres per hour depending on water pressure but would need more careful maintenance.
The 300gpd will probably give you better TDS output than the 4040 hence saving on your Resin costs but it will waste more water in doing so, not so good if you are on a water meter.
Alex Gardiner