A septic tank is always full. If the drainfield will not absorb the water then it backs up into the house.

Houses in our neighborhood built around the same time all have new drywells because of the soil plugging the originals. New drywells here are now installed in pairs with a an A/B switch to change from one to the other on an yearly basis.

The remedy if the drainfield is plugged is a new drainfield.

If it is the drain field, why is only one toilet backing up?

He was addressing your septic tank pumping.

Is this toilet on the 1st floor, 2nd floor, basement...?

And could something other than TP have gone down? Something like a Qtip might be lodged in the pipe or in a bend somewhere.

This is a single story ranch house. Did anything else go down the drain? Its possible, the prior owner appeared to be hard on the house and she was a she with a daughter so, well you know what could have down the toilet and be hung up somewhere. As far as my wife and I we are pretty strictly just a TP household and my little one can't flush the toilet....yet.

Pencils in the toilet create such problems. Toddlers with pencils put them almost any where.

You need to remove the toilet and look in the trap from below - it sounds like you have something lodged in there, and it could be any number of things. Plunging or snaking will generally not remove such items.