I have a old single floor house. I replaced my piping from my bath sink, tub/shower and toilet. I have a old septic tank. (concrete square 2000 gal.) I am having field line problems and the tank is full. This didn't happen at first but now my toilet bubbles. Can this be caused by venting? Or is it going to be septic tank?

Bubbles indicate a partial; blockage producing the back pressure that makes bubbles while a gurgle indicates a vent problem. I don't think you have a venting problem.
My guess is that the back pressure is coming from the septic tank. Good luck, Tom

I agree with Tom's assessment, bubbles m,eam blockage. Septic tanks are actually always full. That's how they work, they fill to the top and then over flow to the field. I seriously doubt you have a 2000 gallon tank if its old. Most were 1000 gal. at best and upgrades were 1500 for a 4 bedroom home. When was it last pumped? When do you see the bubbles, just when the shower is used or what?

I agree with both post. Same problem I had last year. Pumped the septic tank last year ($225.00) and no problems till this year I had a drain blockage from soap scum which was cleaned out today with a snake. I would definitely have your septic cleaned out (pumped) ASAP... It will only cause problem later on like I have. Spent lots of money on do-it-yourself but from first hand experience call around I found a guy who did it for $50.00 cheaper because I told him the other quote I got.. Good Luck