PSA: A plunger can help.

[WileyS]

Can you use liquid plumber or some other liquid?

[WhittleBurner]

Can you use liquid plumber or some other liquid?

I would think the chemicals might be to caustic on the seals, stick with lots of water and patience

[MoonShadow_1911]

I would recommend a product called Bio Clean (you can get it on Amazon).

4 teaspoons mixed in warm, not hot, water, pour it into the bowl, add more water, then flush the whole thing. Leave it wait for at least 24, preferably 48, hours, dunno the tank and repeat with just water in it, filled about 1/2 to 3/4 full.

I alternate between Bio Clean and Unique.

Also, when traveling, I dump before I leave, use Bio Clean and 1/4 tank of water then when I get to my destination, I dump. Kept the black tank running well so far. The biggest key is water is your friend. When you flush a number 2, hold the pedal down for a slow count of 10 (about 8-10 seconds). 5 count for number 1.

In the summer, or when it's hot (> 90°), dump the black tank every two to the days and use more water. It helps keep the oder down.

[Captcolour]

Reading all of these threads, there are different reasons for the clog and different places it happens depending on your specific RV. OP has a bend in his pipe to the black tank which is the cause in his case. Same for me with the master bathroom toilet in my 2021 380FL. Does not take much to clog the pipe going to the black tank. Even stressing using lots of water and a little TP, it happens. I just run the foot pedal until water backs up into the bowl, then use a plunger to clear the pipe. Works great. I can hear the difference in flow in the pipe when it is clear and when it is clogged and going to back-up.
Other posts are about TP teepees in the black tank itself. Different issue than the OP.