For freshwater, you need a food grade tank, period.
Don't cut corners on this one.
You don't want to slowly poison yourself.
Think of all the money you're saving by building your own skoolie rather than buying an RV. Don't be so cheap that you end up hurting yourself and your friends and family. Just get a food grade tank They're not that expensive.
I got this one and it was well worth the $118 to make sure we have safe drinking water:
That one was great because it's rectangular and fit very well under the bus. In another build, I used a food grade 50-gal round blue barrel tank, worked just fine. Cost something like $15-25 locally, used.