You might consider a Flojet macerator instead of your portable 33 gallon black tank system. For about $150 or so and a couple of old 3/4" garden hoses, you can pump the gray and black water from your trailer to the septic tank cleanout. It'll be much more "sanitary" than spilling a load of black contents as you dump and much "lighter on the back" not having to push/pull/lift/wrestle that 33 gallon monster.
Here's a description of what I'm talking about
In the video, he just "pushes the garden hose down the sewer outlet"... I'd suggest buying a twist-on fitting and a 3/4" garden hose cap so you can essentially have a "closed system" from the trailer to the sewer cleanout.