If you were going to consume that water I wouldn't be tempted to use a fuel tank. Food grade plastics are different to fuel grade.
You can buy caravan water tanks as small as 30 litres and should be able to bracket them up under the tub. Run two hoses out - one to the top, one to the bottom. The tanks have an overflow built in. The lower hose can have a small (again, food grade) water pump attached so that you can deliver the water better than gravity allows.
Alternatively, get someone to custom-build stainless tanks for you. It's just a sheet metal job.
The advantage with removable containers like you are using is that if they become contaminated, or are otherwise rendered unusable, they are very easy to remove and clean and cheap to replace. Caravan tanks or custom tanks are not, on either score.
If you choose to use tanks, you can try what we do: our caravan has a pair of 59 litre tanks. At the end of each of our weekend trips, I choose the alternate tank (front tank this trip, rear tank next trip) and I pour a fair amount of baking soda in the tank for the trip home, turning on the taps to get some of that up in the pipes. This leaves fresh water in the other tank in case I have a busted radiator, or whatever other reason. When I get home, job #1 is to drain the tanks. I then flush the treated tank (and associated pipes by running the pumps) to ensure that as much as possible of the baking soda is gone.
The water in my van tastes as good as it comes out of my tap. I don't use a garden hose or one of those so-called "food grade" hoses - they're garbage, and you can taste the plastic in the water as it comes out the other end. I use a canvas hose that rolls up on a reel, costs about $130 at a good camping store.
