you will soon get fed up carting them 25 ltr containers about, i did , you say you dont want to be tied down to your van , but with the trolly you will be back and forth to the van all day , lifting containers in and out of van.
your best bet is a 300 ltr van tank , and a back pack ,
i have a shurflo, had it for around 5 years, and a 500ltr van mount, my pump filter and varistream are mounted on a piece of upvc fascia, which is screwed to ply lining on side of van . with quick fit connectors, the whole system can be taken out in less than 10 mins, for times when i want to use van for something else, in winter i can take out pump so it does not freeze.
daz