If it really old, probably dynamite??? Just kidding. Since your not gonna be moving it, you will probably have to treat it, let it sit, treat it again, let it sit, etc. Somewhere on this site, this same subject was beat to death....so, look for all the answers on that post. Lot's of good/great answers.
Paul1 flush your tanks after you dump them and then put 4 caps full of Calgon Water Softner and about 3 gals water and drive around awhile and let sit overnight. This works on my 1985 Class C Shasta 29fter. I do this with both tanks and its worked for me since it was new.
A guy at an RV store told me that the way to clean them is to take two alka seltzers and toss them into your tanks and voila, they scrub the little sensors clean. Haven't tried it but he swears it works.
As you may guess based upon the variety of responses, cleaning the interior of waste tanks is mostly a matter of opinion and there is not really good answer. Just about all of the suggestions that people make will help, at least sometimes and for a while. But in 30+ years of RVing and five fulltime, I still have never heard of anything that is really reliable, every time for everyone. And that is with the RV movable. Movement is important because the movement creates agitation that is a major part of the cleaning action for nearly all methods. Since you do not wisht to move the RV, there really isn't a lot that you can do. And I would guess that you really do not wish to spend the $300+ that it would cost to put in the level sensors that go on the outside of the tanks. So now we have a very limited range of answers.
It would be my guess that your best shot would be to get some of the bacterial tank additives and give each tank a healthy dose each time you dump and then allow the tank to fill completely before you dump. That may help. One thing that can be done is to keep the dump valve to the gray tank open all of the time, except when dumping the black. That will help since you won't care then what the gray tank level says. And you can look in the toilet and see the level in most RV black tanks, since the toilet is normally directly above the tank to allow for the contents to drop streight into it. But do not be surprised if you never find a really good answer to keeping the level indicators working accurately.