This topic comes up on a regular basis, and since I was where you are about a year ago here are my thoughts. The problem with a retrofit grey water tank is the shower pan. It's at floor level. Notice your black water tank is at floor level also. To drain shower water into a grey water tank requires that it be fitted about five inches lower than the shower. Why? Because there needs to be a p-trap plumbed under the shower pan, and that plumbing takes up a minimum of five inches. If you replace the factory p-trap with standard hardware store fittings the p-trap will need about 7 inches. The space beneath the floor of your '68 AS is only about five inches between floor and bottom aluminum skin (that is between the framework of the chassis). So, the choice for placement of a grey tank puts it under the belly (beneath the chassis that is). Or, you can raise the shower pan up 5 inches and install the grey tank just under the plywood floor in the chassis.
An alternative is to have a small grey water tank just for the sinks. Since they sit high up on the counter tops there is plenty of room for plumbing the drain to a small tank that might be installed at floor level, perhaps under the bathroom sink cabinet (sitting next to the hot water heater perhaps). A Bladex waste valve would have to be installed between the grey water tank and the shower pan (otherwise the grey water will back up into the shower pan when the sewer outlet is capped).
Go look at some other brand of trailer. You'll notice that the showers are the step up type and have a dome over them to create head room, or they have a "basement" where there is room in or under the chassis to drop plumbing down to a holding tank.
I decided to stay with the original design and carry a "blue belly" with me for those stays where draining grey water onto the ground is prohibited (and enforced). It was simply the more expedient and cheaper way to go. My experience so far is that most times you'll be able to simply drain the grey water onto the ground, off into the brush. Most places out west need the water anyway. Some people get real upset about this practice, saying that nasty bacteria is drained onto the soil that can then be ingested by their children when they eat dirt or mud pies or whatever (I'm joking - a little). So, do use some consideration and try to extend your grey hose off into the weeds or brush where no-one is likely to sit, walk, or roll around.