RV holding tanks come in many different shapes, sizes and drain locations. How well a "Blaster" will work depends on your particular tank. Effectiveness can range from zip to "sorta/maybe okay on a good day if lucky". The first tank below for ex. has a recessed end drain and the spray would hit the opposite wall and do nothing.
The second tank has an end drain and the water would have to hit the opposite tank wall 5'+ away. Note the location of the tank sensors. The spray will never hit them to clean them off.
And then you'll have one or two bends in the drain pipe so there really is no direct water "spray" hitting the innards of the tank anyway.
Our black tank is like the third photo. Again, there's no way a spray would clean the sensors off. We have a back flush elbow that worked reasonably okay on a previous TT (had small tank) but does nothing for our current TT and the photo explains why. We have an in-tank rinser but the spray nozzle is on the same wall as the sensors so does little if anything to clean the sensors. I let our in-tank rinser run about 15 minutes or so while breaking camp. When peering down into the tank, the tank bottom gets pretty clean but have no idea how clean the sidewalls or the tank bottom elsewhere gets. We even have Horst Miracle probes that don't help and was money wasted.
A toilet wand sprayer is as good as you can get but not exactly convenient to use.