Assuming your Agile is similar to my SS Ideal the charge may be reasonable. Yes, the def tank is located mid way back, above the side door step, grey water and fresh water tanks. It looks like a factory location vs Roadtrek modification to me.
My def pump wasn’t producing adequate pressure - def pump had to be replaced - step, grey and fresh tanks had to be removed to access the def tank / pump. My MB shop wouldn’t remove / replace them so I agreed to do so myself. After raising the van 18 inches and about 2 1/2 hrs work I had the items removed. MB replaced the pump and I spent almost two full days replacing the offending items and related content level sensor electric lines, hoses, shields etc. Roadtrek used every bit of space for custom built grey / fresh tanks leaving no space for worker hands.
I tried to insert a small tube / hose down through the def fill line to pump out the def but couldn’t make that work. Removing the tank may be the only way to drain it. Puncturing the def tank, draining and patching the tank may be an alternative but removal of step, grey and fresh tanks would still be necessary to get to the def tank. I can believe the MB technician could spend the hours to equate the $1100 charge.
Similarly, years ago when MB shop (different one) had to replace Nox sensor or some related part they found they couldn’t access the sensor again due to the fresh water tank. Luckily the technician figured out that he could remove the inner fender liner and access and replace the sensor much easier than removing the tank.
These experiences demonstrated to me one of the reasons these RV conversions are so expensive - custom design and build of tanks, plumbing, wiring, etc to use every bit of space possible and then next to impossible work to install.