FIrst off, the normal state of a septic tank is full, but it should only be full of liquids if it was pumped 15-days ago. The liquids and solids go into the tank, the solids gradually sink to the bottom and accumulate if they don't get decomposed, and it is when the solids build up that you need to pump the tank out. The clear(er) liquids then overflow the tank into the distribution box and then go into your leach field...so, some in, some out with each use. If you have a drain backup, it could be that the leach field has failed and can no longer absorb any liquids. This can happen if you haven't pumped the tank often enough and both liquids AND solids get washed out which will eventually plug up the field. Otherwise, there's likely a blockage going INTO the septic tank giving you grief in the house. SO, it may have nothing to do with the state of the tank and leach fields at all, only the pipes to it.