I had a steel trailer once, and the previous owner had put in a home made dividing wall to make an LQ.  He did it in wood, and then on the horse side installed a removable chest bar (it was a straight load) so the horses had head room and no contact with the wall.

In the LQ area, I sistered 1" x 1" to the studs, which are never equally spaced along the trailer wall so be prepared to measure, measure, measure.  Styro insulation between the studs, and good one side 1/2" plywood for the walls, prepainted BOTH sides before install.  Did the same treatment on the ceiling, but for the curve between the top of the wall and the ceiling we cut strips of a plastic board - I can't recall the name but it comes in 4' x 8' sheets and is commonly used in barns around here to sheet the base of the walls.  It's quite flexible and we only had to bend and tuck it into place.  All the wiring for the trailer lights ran behind this, so it was easy access and looked really good too. 

The floor was done in linoleum, built an L-shape bench with storage below that converted to a bed, and had a removable triangle shaped table that stood in front of it.

The walls of this space were 10' long, and since I had no tack room, 4 saddle racks were installed on the dividing wall.  All this and still held a family of 4!

We did no plumbing, so can't help you there.  One show we went to I remember it snowed and we were in there pretty toasty with only a space heater that we had to keep shutting off so we wouldn't bake.

A friend of mine bought the same trailer I have without a weekender package. (Exiss 4' weekender Sierra) My trailer weighs about #900 more than his. We built a weekender LQ in his trailer, copying the floor plan and accessories that mine have. We installed full plumbing, fridge, cabinetry, water heater, enclosed bathroom, insulated floor, walls, ceiling etc. We did eliminate the installation of a gray tank, using a black water tank for everything.

The total weight gain was again about #900. Almost all of this weight is near the front pin of the trailer, with less being placed upon the axles. A standard guess about pin weights on GN trailers is 25% of the total. This actually greatly varies by dressing room lengths, equipment carried, ST loads VS SL, and importantly, axle placement. The addition of a LQ skews these figures considerably. The pin weights can then be almost 40% in some circumstances.

If you take your axle capacities and add 25%, you will end up with a theoretical max carrying capacity. If you subtract the weight of the trailer added to the loads, hopefully you will end up with a figure that is less than its max capacity. This is commonly referred to as a reserve capacity. The higher the reserve, the less stressed the trailer's structure and running gear will encounter.

Example; a trailer is built with 2 3500# axles giving a max of 7K capacity. If the pin supports 1750# of that, you have a maximum possible capacity of #8750. If you carry three horses and gear weighing a total of 3500#, and your trailer weighs 4000#, you will have a reserve of 1250#. Adding a LQ will lower the reserve capacity. By how much? Only by physically weighing the truck and trailer can the actual weights be known. There are just too many variables.

Some trailers with elaborate LQs, are built with almost no reserve capacity. If you hear of a 17500# trailer being supported by 7K axles, you will understand that the manufacturer is depending on the pin, to aid in carrying the load. This trailer will have little or no reserve capacity. You will hear of frequent axle problems and tire blow outs. The various components are stressed to their maximum and will eventually fail. A trailer built with a reserve capacity, will provide a longer trouble free life span.