I would do it all DIY, but I would spend money on a custom made tank to maximise my vehicle space and to suit my exact set-up, filling lid position (very important if filling your vehicle on a slope), outlet position, then I would get a local firm/garage to make a bespoke tank frame for it (or bespoke mounting eyes fitted for ratchet straps) . I would use a small 300gpd RO unit into a second-hand IBC. I would use a Flojet pump and 100 metres of microbore.
(I went to a big firm 6 years ago and paid comparatively lots of money

, but it at least got me started which was better than sticking to ladders

). When I started there was not the back up for DIY'ers and there were fewer suppliers.