I was looking to add some extra water storage so I tried fitting a 15 gal water bladder in the spare tire location.
I used a plastic mortar mixing tub to contain it and strapped it to the frame with ratchet straps.
Where there was no easy way to anchor to the frame, I used Unistrut channels secured to the bed supports with Riv-Nuts. I think the full tank is about 130 pounds, behind the back axle but about the level of the cab floor.
Matching the opening on the tub is a plywood top (also attached to the frame) to keep the bag from moving around or out of the tub. The rectangular shape of the tub left me enough room to mount a 2.5 gal. air tank under the bed as well.
The whole arrangement is about 8" tall, so it is well protected and still flexible enough to endure scrapes and dings.
There is a hole in the plywood lid to access the fill tube and a hole in the bottom of the tub for the drain tube, which I can use to pump the reserve into my main water tank or empty into a bucket.
Filling is not too difficult- I attach a small hose to the drain and then secure it just above the hight of the top of the bag. With a hose on the fill tube, the bag will fill to the hight of the overflow without bulging too much.
With the fill tube opened as a vent, the bag drains at a reasonable pace and most of the water stored can be emptied out and used without lowering the tub.
The whole setup did over 3000 miles in a 3.5 week tour of southern Utah- rocks, washboards, deep sand and gravel roads without issue or damage. When I dropped the tub, there was a fair amount of silt, I imagine a good deal of dust found its way in and a few river crossings turned that to mud. The bag seemed unaffected and is still watertight.
The extra water let us stay out longer, take showers and be a little more relaxed with our water use.
The bag was fairly expensive, Aquatank2 it is called, $95 for the 15 gallon size online. The first one I ordered had a small seep around the fill tube, but the vendor sent me a new one right away and it has held up fine.
The other materials were on-hand or inexpensive, but getting the Unistrut fit in a plane and attached was a bit of work and making sure the fill and drain tubes stayed accessible while being protected also took time.
