Idaho Joe wrote:
After further thinking I will more than likely just modify the pipe to drain into the black water tank somehow.
As noted, if you do that be sure to install a trap in the sink drain to prevent the black tank from venting up through the sink. Also be mindful of capacities and how you plan to use the trailer. If you are boondocking with no facilities and you fill the black tank up with gray water, that effectively takes the toilet out of commission. where you might get away with dribbling some gray water in the bushes, you won't want to be dumping a whole tank of black/gray when it fills up.
Not saying it's a bad idea, just a few considerations to think about. Depending on the underfloor space in the trailer, you may be able to add a gray tank and plumb the sink/shower into it. You would still want to trap those drains though and also vent the tank.