You may have something in the tank that is moving towards the outlet when you dump. That could be a tank cutout that wasn't removed, a kid's toy, or a wad of something that blocks the output. I'd suggest to put some soap, water softener, or digester in the tank and drive to a dump station. You don't need a flushing hose to dump.
While you're waiting for the tank additives to work, installing a couple of backup lights so you can back up at night might be a good idea.
A macerator pump may be a good thing to have for your situation, too. With that, you can dump at your house and not have to move.