You need an upflush toilet.
The drain is plugged between the tub and the sewer or septic or in the case of the septic system, the tank may be full. The toilet had to go somewhere and the tub was available.
My cats play with toilet paper. They scratch it, unroll it and roll in it. I can't get them to stop, but they don't eat it. You have a strange CAT. Septic Safe toilet paper is recycled paper with no chemicals. Should be safe to eat as long as it hasn't been used in the toilet or septic tank.
That would depend on how far it was to the septic tank.
wrong stuff placed in the toilet
There may be something that is partially blocking the drain.
Toilet / piping to and from the toilet and sinks and tubs and showers
An RV toilet is designed to be mounted over a holding tank. They use almost no water which at first might sound appealing, but unless you have a septic tank under your bathroom it would be bad news for home installation. Unless modified, an RV toilet would not be able to get the solid waste to your home septic system. A home toilet uses the force of the water to move the solid waste to your septic tank. Look for a low flush toilet or a composting toilet instead.
If you really mean it flushes but the water does not run away, then the septic tank is plugged.
Proper monthly maintenance of your Septic tank is necessary to keep it in proper working order. There are several products that are available. Rid-X, Bio tabs, and Flush-It are the leading brands. All you have to do is flush them down the toilet each month to keep your septic tank working properly.
Drain line clogged. Toilet itself may be problem.
This all depends on the condition of the septic tank. Its like asking, how long is a piece of string
You need your septic tank cleaned out.