LarryJB gives the best reason (to me) of why not to combine both into one tank - most RVers use the grey water to flush the hose system after dumping the black tank. If all combined, that part of the flush is not possible. If there is no rinse water available at the dump station (I have found about half of the dump stations we have used don't have flush water available), then in order to flush-to-cleanse, one would have to go inside the trailer and run their water pump to put more fresh water into the combined tank, and hope you had enough fresh water available to do a good flush of the tank.
Regarding the RVs being made with combined tanks, all that I have seen have the bathroom sink, and sometimes the shower, going into the black tank, due to tank placement, or balancing of water discharge. They all have another grey tank that takes the kitchen drain, and most take the shower drain. This way, there is always plenty of water in the black tank to dissolve the solids. I have not seen any RVs that only have one tank for both black and grey, combined.
|