So, the old septic tank has to come out, and you need to relocate the tank. It sounds financially painful. Actually, it does not have to be. If you can get the septic installation contractor to mark off the area where the new tank has to go, you could DYOSTH-- Dig Your Own Septic Tank Hole. Here is what that would look like.
With the new tank area marked off, you need to grab a spade and several digging shovels to get started. Push the spade into the earth all the way around the perimeter of the marked off area. This shows where you have to stop digging as you start digging downward.
If you park a mini-front loader near the hole, you can toss the dirt into the front loader. Then later you can move the mini-front loader with dirt to where you can dispose of the dirt. (It helps if you have at least two helpers in this endeavor; two extra people to help dig, and one can jump out and operate the front loader each time it gets full.)
Dig to the Specified Depth
Your septic tank installation contractor will tell you how deep to dig too. Keep a meter stick or a pull-out tape measure on you to measure the depth. It may be deeper than you are tall, but if you have helpers, you will all be able to climb out of the hole safely.
Even out the Sides and Bottom
Your new septic tank has to sit on a flat surface. This can be difficult with mud and certain soils. If the bottom of the hole is fairly uniformly the same depth, use wood boards to tamp down the earth and make it nice and flat. These could double as boards to help you climb out of the hole when you set the boards at an incline against the sides of the hole. As for the sides, you can use a flat-edged, blade-like mason's trowel to get perfectly cut sides.
Cover the Hole When You Are Done
This project could take most of a Saturday. When you are finally done and you have climbed out of this hole, you will need to cover it with a big tarp. Use bricks to hold the tarp in place over the hole so that the tarp cannot be blown away in a storm. This protects the hole and prevents excess water from filling it until your septic tank contractor can bring the tank and install it.