Any tank with a sag or belly in it is NOT going to empty properly or completely. The inevitable sludge build up from the ordinary digesters is going to add further weight to the sagging tank. The sludge will accumulate to a higher level than the water level and will still not drain because it is too thick and heavy. This condition may even compromise the tank to the point the tank breaks free from its supports and falls completely out of the unit. The extra weight of the sludge plus extra volume could take it beyond its designed weight capacity. This is more common than you would expect. If the tank falls out on a freeway in some states, there could also be a hefty environmental cleanup bill. Seriously, it has happened. The best solution is to have the tanks supported properly. In the meantime, it is imperative to use TankTechsRx in the holding tank to allow it to consume the sludge allowing more water to empty and lighten the burden on the supports holding the tank. Full tanks with bellies should only be driven as far as the nearest dump station. Use TankTechsRx with every tank to minimize the risk.