Another form of rejuvenation is called the terralift,it is a machine with a probe that blasts or hydrofracs the soil under field to loosen it to let field percolate,that can cost 3500.00. The price you are quoted for jetting can be high because they may put in chemicals after and a septic truck has to pump out the material the jetter loosens from in the lines. I have jetted fields,installed effluent filters in tanks and added vents to the leachlines to add oxygen to field to keep it from suffocating. If your tank is slowly receding ,if it gets to 12 inches below the opening in tank,that should be the normal height,the pipe to leachfield leaves the tank at approximately 12 inches from top. Some older tanks have cast iron elbows in place of cement baffles,I've had those plug up with rust. When your pumper pumps your tank,he should hear water running back in from field if it is flooded,if not it could have a problem getting to field. I use a telescopic mirror and flashlight to look in tank after it is pumped to check condition of tank,baffles. plowking