The tank will always be full to an inch or so from the top. The discharge end is typically 1 or 2 " below the inlet.
The pipe in your first photo looks like the discharge end. The bottom of the pipe should be in the approximate middle between the top and bottom, which appears to be the case. The reason for this is that the tank collects stuff that either sinks or floats, so the cleanest water is in the middle. Normally though, there is a “sanitary tee” connection that makes it easy to service.
The bad news is that the tank was probably never serviced. Normally they should be pumped out every three years. If not, either the sinkers or the floaters get into the discharge, then on to your leach field, which then clogs. Once that happens, especially if the floaters got in there (grease) the field is probably history.
You’ll need to call up a specialist to make a repair. If the house is less than about 20 years old, depending on where you live, there should be an area for repair set aside somewhere on the property.
Since this happened so soon after you moved in, I suspect that the seller knew there was a problem and didn’t disclose it. Get your lawyer to file suit and discover his maintenance records.
When you do make the repair, have the installer put in an effluent filter on the discharge pipe in the tank. That will keep sinkers and floaters out of the leach filed altogether so this doesn’t happen 3 in the future.