Given the time that has passed since you received the initial responses/advice, I'm sending this to your direct e-mail also. I am currently restoring a 1968 22' Safari (rear bath w/ tub) and, of course, it has no gray water tank as was the standard for that time period. My Safari's three drains converge in the "crawl space" under the tub above and road side of the black tank.. That is to say that the galley drain (also street side) drains to the rear where it merges with the tub drain where they share a common vent on the road side of the trailer between the walls which form the area that accepts the retracted pocket door to allow entrance to the bathroom.
The third drain is from the bathroom sink and, in what could only have been engineered during a time of great prosperity by one who's family holdings included vast quantities of ABS pipe, utilizes no less than 25' of said pipe to vent and drain that little street side vanity forward and then aft along the sidewalk side wall until it sweeps across the back access compartment to where it converges with the other two drain lines just prior to their now common line terminating just after the knife valve for the black tank (and which thus, at least at that point, seems similar to that you describe in your trailer).
Since I am replacing the aft 4' of sub flooring in my unit, I have noted some possible locations for a gray water tank or tanks. First, the tub floor is elevated nearly six inches above the sub flooring allowing for a trap, merging and drainage. The tub thus sits atop a plywood box which gives the tub it's "floor" and the required room for the three drains (and one vent) to meet. A taller space in the aft road side corner is also used for the merging drains as well. This space is also used for the city water inlet, pressure regulator, and as the main branching for all hot & cold water lines. From here the cold line feeds the tub and toilet on it's way to the galley in one direction while sweeping across the aft access compartment to feed the bathroom vanity and hot water heater in the other direction. The hot water return then feeds the hot side of the vanity before it sweeps across the aft compartment to feed the tub and then the galley. Note, the hot water feed line sizes down from 5/8" O.D. to 1/2" O.D. at the point where it continues to the galley after the tee to the hot side tub faucet. Also, all tees supplying hot or cold to faucets & toilet accept 3/8" O.D. copper for the uprights.
My thoughts/plans are as follows:
1. I will redesign the boxed up floor for the tub so I can add a gray water tank in that location under the tub. A significant amount of space for this tank already exists under the tub and additional room will become available due to the fact that the "Y" or Sanitary "T" merge for the galley and tub drains, as well as the "U-turn" and a second Sanitary "T" merge of the vanity drain in the street side aft corner can be eliminated by instead terminating each of the three lines into the gray tank. Rather than a train yard configuration of merging drain lines, each line simply feeds into the new gray tank. Also, as the new gray tank is above the black tank and rests on the same sub floor the three gray drains once did, the drain for the tank will pick up the last portion of the gray drain line just before it terminates at the black tank knife valve and thus allowing for a gray water knife valve to clear out the dump line beginning at the Sanitary "T" immediately after the black tank knife/dump valve.
2. I have not calculated the capacity of the new gray tank and may find it to be inadequate. This is especially possible as I intend to keep the tub drain's "trap" which will result in a smaller gray tank than if the tub were simply allowed to drain straight down into the new gray tank. If the capacity is too small, I have considered installing more "gray" tanks in up to three other locations. First, the street side aft corner could accommodate a much taller tank to catch only the galley and the vanity drains. This tank and the lower profile tub water holding tank would then simply tee together at the point they both joined the dump line. To prevent galley/vanity waste from overflowing into the tub I'd put two knife valves entering that tee. One for the tub waste tank and the second for the taller corner tank.
3. Secondly, the sidewalk side aft corner could accommodate another "tall" tank (in place of the vanity drain's 45 degree drop to the floor of the aft access compartment. The vanity will drain into the top of that tank and a knife valve would allow the tank to dump into the line currently swept along the back to the merger point with the other drains.
4. Finally, a similar holding tank under the galley sink could be installed to retain that waste water as well.
I don't know how far you've torn into your trailer so some of this may seem overkill to you. Since I had 4' of sub floor to replace anyway, I'd already taken out the entire bathroom from the bulkheads aft, including the divider between the bathroom and the aft access panel as well as the hot water heating unit and tank. [It appears as if a leaking hot water tank was as responsible for the floor rot - and termites - as the toilet may have been.] So for me, removing the drain and water lines is easier than trying to place the new plywood sub floor with them in and, once they're out, I may as well reinstall them into (a) new gray tank(s). [The feed lines will also need to be moved to accommodate the new tank(s). But the result of that will likely give me more storage space than I currently have in the aft corners and the aft access compartment!
I also have yet to determine where to buy the new tank(s). I considered the PVC or ABS looping as someone else described, but rejected it as too much space and material for not much holding capacity. I will look into marine fuel tanks (4" profile are apparently standard) as well as going with (a) custom made unit(s). I will probably lean to custom as the tub is more triangular than anything else. If you're interested in my efforts, I have pictures and will be moving forward as described and would be glad to share them with you or anyone else. Please don't ask for me to post the pictures, however, as I fear that such a task will entail a learning project that I will not engage in until after my trailer is camp ready!
Good luck and I hope that this long winded message was the result of accuracy and clarity and not because I like to hear myself type. If I can be of any help, you may contact me off-list if you care to.
Richard Higgins