[harleyman]

What transfer pump do I need to transfer water from  water barrel to my tank in the van  bearing in mind it has to go through my van  filtration  built in  I'm  running out of water on some days and don't whant to start using customers out Side taps.

[Spruce]

What transfer pump do I need to transfer water from  water barrel to my tank in the van  bearing in mind it has to go through my van  filtration  built in  I'm  running out of water on some days and don't whant to start using customers out Side taps.

We are a bit confused by your post and this is probably why it hasn't been responded to.

Please explain about the water in the water barrel. Is it collected from the roof? What is the built in filtration in the van?
If the water is from the roof (rain water harvesting) what bits are in it or have you filtered them out before the barrel?
or
Are we talking about water in a barrel (butt) that has been filtered with r/o and you just want to polish it off before it goes into the van's tank?

[Plankton]

I'm a bit confused as Spruce has replied "We are a bit confused" Is there some VIP page where you all discuss the goings on!🤔
Other than that I'd imagine your running di before your tank. I've recently changed to this and will be looking to change back to di after tank as it gives greater flexibility.