and here is some more
Finally got the water storage and waste water tanks. I ended up using Plastic-Mart. They had reasonable prices and I found nearly the exact sizes I needed.
The water tank is just a basic unbaffled 20 gallon tank. All fittings are customizable. I just put a vent, drain and fill fitting.

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The tank was fitted up for marking holes then temporarily strapped up with plastic plumbers tape.
For the waste tank I decided to use a standard RV drain valve. Time will tell if this was a good idea. It sits pretty tight up to the frame behind the spring shackle. This tank came pre-molded with a 3" drain and is tapered at the back, just enough to tuck up to the rear frame member. I had a 1 1/4" fill and 3/4 vent fitting added. I got a 3"-1 1/2" adapter and the valve at the local RV repair store and the ABS fittings at the hardware store.
I will get some 1/16 or 1/8 bar stock/strap, add some bends for soft corners and bolt it up to the frame using Rivnuts. This is how my Land cruiser gas tank is mount so I think it will be adequate for the trailer.