How far is the septic riser from trailer?
I have used 3 sewer hoses (20 footers) to connect and dump my tanks........even if you have to walk the hoses after dumping it's alot easier than repeated trips with tote.
And the clear adapter...priceless because you can see the flow and watch how well it is draining.
If it truly is only partially draining....then either cut out disc was left behind (not an uncommon thing) and gets sucked down to outlet blocking drain or some other foreign object.
It isn't poop, TP etc........has to be something solid to block/stop draining thru 3" line. Water is heavy and will push boulders out of the way.