I got 2 - 15 gal plastic food grade barrels for $8 each at my local market. I'll post pics when I get outside this morning. I am debating weather to use both or just one. These are the type with the latched lid nice quality.
Look for a camper & RV parts store around you. Picked up a 32 gallon tank made for this application for $80.00.
They have several different sizes available to fit your needs.
It has to have a fill line, vent line and drain line.
I'm almost certain that Tractor Supply used to have something that would be legal, but not positive. Several years ago I picked one up with a buddy to replace a leaking tank in a horse trailer w/living quarters.
This is what I picked up for my needs. They are a little covered in pollen from my tree but they were clean when I picked them up 2 weeks ago. Used for syrup in making food. The lids are removable which makes them nice if I ever want to clean them. If I use both, I'll plumb them together with a valve in the middle to control how much water I need to carry - install a vent on each - and a fill on the one with the pump. I see this as a better setup for a tiny trailer to reduce sloshing should I haul them full. Hope this helps being that its a $16 - 30 gal tank(s) setup. (I will not be stacking them in the setup. side by side)