For state parks w/out sewer hookups I bring-along a 42 gallon Barker waste tote. The tote has wheels & a handle that fits-around the hitch on my truck. When it is full I just slowly tow it behind my truck to the dump station.
Since your TT only has 12 gal. capacity, you can leave it under your TT with gray valve open for longer showers, etc...
And then, we have a 42 gallon FW bladder to transport more FW to our TT when we are not close to a spigot... Since we have a fiberglass cap, I just place the bladder on top of our cap & fill-up with FW on the way back from the dump station.