This has helped us as much as the "Geo" method if not more. It wasn't intentional at first but to help with the black water side to save space in gray water when boondocking. We've noticed that the tank gets a lot cleaner(past several years). We used to put dish water soap in black water tank when boondocking to save in the gray water side(space wise) but now we always put dish water soap down black water side. I guess it splashes around and clean things up when driving around; were happy with results. We do the "Geo" method about every 3rd or 4th tank but always put dish water in toilet and flush it down. By your posting description
will it help loosen build-up?
maybe you have to much build up. If to much build up is the case I would answer with what you don't want to hear (water then drain, water then drain...) until you get it under control. If you do resort to a pressure washer, I would use a garden hose with nozzle first. Blowing a hole in black water tank would not be cheap to fix.
The worst part about us putting dish soap down black water side is the size of the plastic pan that fits into RV sink. Ours came from Hosp stay, those little cheap patient pans things fit our RV sink the best, because of odd ball size of sink.
If you do resort to pressure washer(last resort), use the widest nozzle you have, like 40 degrees or more. And get a clothes hanger wire and give yourself maybe 15 inches from the tip to end of hanger and tape other end to the washer near nozzle. That way the hanger will work like a guide and don't get pressure washer any closer than your hanger overhang (15-18"). I assume you're talking about a 2 or 3 thousand psi pressure washer and not a 5k or bigger. But over time the water will do the trick. (don't forget your haz mat suit):B