I would contact the trailer manufacture and get a copy of the build sheet. It tells you where they framed for the tanks. Usually pretty ease to tell as they typically leave out a few I beams to allow room. It would be unusual but if they didn't leave out the beams, you can put tanks about anywhere, but it they would hang down more than normal and could present problems dragging them. Our first trailer, we had to cut part of the beams out to get the tanks up in the floor and that was a major pain and wouldn't recommend that.

As far as hanging them, pretty straight forward. Where you buy them should have the staps needed.

Good luck.

Because the floor joists are so close together, the installation of a tank of sufficient size, will involve the removal of at least part of one joist. The entire joist does not have to be removed, just the vertical web and the bottom plate. The top plate is left in place to contribute stiffness to the flooring. We used several different tools that cut an aluminum joist. With steel joists, it might be cost effective and a labour savings, to have a fab shop use their plasma cutter to effect the cut.

Depending upon your intended usage, you may want to consider only the installation of a black water tank, omitting the gray tank. When we were installing a LQ in a weekender package, we only had the galley and bathroom toilet to drain. The shower was installed as a cowboy usage in the first stall. With only the galley sink water to collect, it's drain was connected to the black tank. If you are collecting the drainings of multiple sinks and a shower, a separate gray tank will be necessary.

The tanks are held into place with either straps or flange harware, or a combination of both, which is what we used. A spacer is installed around the tank's perimiter, that is equal to the thickness of the top web that was left in place. We used treated lumber ripped on a table saw. The only way to properly mount the tanks is between the joists. There is not enough ground clearance to mount them under the joists.

Another installation you might want to consider if you desire to simplify things, is using a toilet style found in truck campers. Some are stand alone units, having their own integeral tanks and are designed to be surface mounted on top of the flooring.

Accessing the internet, you will find many sources of the tanks. They list the available sizes and construction material. If you purchase the abs tanks instead of the polyethylene materials, you will find the installation and subsequent repairs/modifications to be much easier.

I would not want to cut out the flooring of a trailer. Most are engineered to provide part of the overall strength of the "box", and are a structural element. This is especially true of a plank flooring system, that consists of welds for each plank at each joist. For instance, a sheet flooring such as 4 Star, would be less stressed than a plank system built by Exiss.

It doesn't take that much work to properly install a tank, and if done correctly, will often last many trouble free years. The only floor cutting for a tank is two holes, a vent and fill. They are circular and won't weaken the flooring. Personally, if I were buying a used trailer and found one that had its flooring removed across its width, I would out of hand reject it for my ownership.

Sorry I don't agree on this type of installation. I think it would be a mistake and greatly devalue the trailer.

My background is not one of engineering, although my job included exactly following engineering blue prints. For three and a half decades, as a certified mechanic, I spent time in the fabrication, repair and maintenance of commercial aircraft with their aluminum structures. My experiences are based on the protocols I was bound to follow, as provided by both our government, and the many individual manufactures of whose aircraft I serviced.

My best advice would be to contact the manufacturer of your trailer, and ask for a list of their recommended LQ conversion installers. Talk to as many of these representatives as you can. If at all possible, look for a conversion that has been made for a trailer like yours. If that fails, I would look for a structural engineer to advise you in your quest.

To reiterate, if this were my trailer, I would not cut the flooring across the trailer's width and replace it with plywood. Take a cardboard shoe box, tape all the joints and try to bend it over its length. It is quite rigid and strong for its weight, very similar to structural square tubing. Now cut one of the length wise pieces across its width. Bending it again upward over its length with the cut on the bottom, you will find the box will easily crush into two sections, whilst failing at the cut line.