As a last resort, rubber suit and a snake up the outlet, you have a blockage that needs broken loose. Only way I can think of that have seen work is a water hose adapter on the sewer out let. Close the sewer valve then put on the water hose and adapter, open sewer valve apply fresh water from valve and watch the in side to see if the clog breaks loose. Be careful not too much water or pressure. If if looks like it worked close sewer valve, remove fresh water adapter. Hookup regular sewer hose and see if it drains.
Other thought, are you in freezing temps and just have a frozen sewer line? If so apply heat with hair dryer and melt to free flow.
First of all...you don't have a SEPTIC Tank...you have a HOLDING TANK. You should leave your black tank valve CLOSED until the tank fills...then open the valve and allow it to flush out.
Now you have a pyramid built up in the tank right where it fills. I would suggest you take a water hose (that you are willing to throw away) and shove it down the toilet and hopefully you can break it loose and get it dumped. Then you need to flood the tank several times until it comes out clear.
I had this same problem once!! After Katrina, I had a camper connected to an out board holding tank that the porta let company would pump out, I left the holding tank valve on my camper, open continuous,This caused the liquid to escape my holding tank to fast, leaving behind a pyramid of solids, had to close valve and fill holding tank on my camper, with water too loosen the solids and dump to the outboard holding tank, so after that episode thats what I did, captured into the holding tank on my camper, and did not dump into the outboard tank, till pump truck was on site
You can also get some very hot water and calgon and pour into the tank, via toilet let it sit for awhile then flush it out. I bought used also and this what I did. You shouild have seen the stuff coming out. I now do this before each season of camping. So for so good.
well I guess I should have said,,,, I also do it after each camping season also. I do it 2 times a year, I also have a flush connection that I hook up the hose and do a good job of water pressure rinse after each camping, so I can imagine mine is clean.