Dave,
I have a astravan which i use for work, i was trad and am now wfp, my system was from pure freedom, i have the 300gpd RO and with a booster pump it makes 200 litres in under 5 hours,
i have a 210 litre tank which i think fits the best in the astravan i started with the 250 got rid the 210 fits better, started with the 17ft xtel fibre glass but now have a 24ft carbon fibre facelift pole, I use a tap on the pole which is from pure freedom and i can make the 200 litres last, i have done £200 days with only 200 litres, and thats wfp everything not just tops and trading bottoms
the astravan isnt the best to use for wfp but i use it and i earn good money, i wouldnt bother with a trolley system, it will strain your back lifting 25kg+ in and out of the back 20+ times a day especially with the tailgate on the astravan
my advice would be defo buy a booster pump no ifs or buts, it has nearly tripled my water production,
buy the best pole you can as you will be using it all day everyday, fibreglass is crap they are heavy bendy and wear out fast,
use a tap on your pole to save water, why people dont i think is crazy! without a pole tap you will use double at least which is a complete waste of money, extra fuel to carry it resin to make it water bill etc etc