[JRTJH]

Since you said that you "normally only use the trailer toilet for #1 and use the campground toilet for #2, I'd ask, do you normally leave the black tank (toilet tank) valve open and let it drain continuously???

If you do, and if, for the past month, you've been using the trailer toilet for #2 with that valve open, I'd guess you have a "dreaded poop pyramid"... That's a "mound of poop and paper" that's dried in a pyramid shape, now, blocking your toilet "chute" preventing more poop and paper from entering the black tank....

If you've ever used a pit toilet in a primitive campground or an outhouse, if you looked down the hole, you saw a solid mass directly under the hole... That's something like what you have under the "chute hole" in your black tank...

Toilet paper, when it gets wet and then dries, becomes "concrete"... very difficult to break up (especially with a stick from the marsh out back)… The best thing to do is to spend some time closing the black tank valve, filling the black tank full, letting it sit (to soften the paper) and empty the tank, fill it again, let it sit overnight, empty in the morning and repeat until you soften and break up the poop pyramid...

You might "cautiously and gently" use that stick to help break up the pyramid, but use extreme caution. Your black tank is made of plastic and over the years from 2007 to now, that plastic has aged and has become brittle, so slamming a stick down the toilet and hitting the bottom of the black tank is a good way to break through the plastic and create even bigger problems...

Soften the paper by soaking it with the black tank full...

And: NEVER, EVER, use the black tank with the valve open. ALWAYS keep a good volume of water in the tank so the solids that get flushed "down that chute" have the water to help "float them away from the hole, making way for the next flush"....