First thing we usually do when a camper is new, is time the filling of the tank. This may vary a little from place to place due to water pressure / flow, but for the most part we dump in our shop anyway (Full hook ups). I'll turn on the water, start the stop watch, then go watch from inside through the toilet with a flashlight if possible. On the trailers where you can see the end of the pipe, you'll know exactly when it's full. The fifth wheel proves a little more challenging sometimes when the waste pipe has bends in it. Either way, at that point I know when the "Oh Crap" point is. So if it's 10 minutes to completely fill, during flushing, I'll fill it for 7 minutes, then dump it. I don't trust the tank probes, as they are not always really at 1/3, 2/3 and full.
I'll second what bubbles said about being your only concern. We / I almost had an incident where I got sidetracked. I heard the gurgling of the vent pipe about the same time my wife hollered out the door (She was cleaning bathroom). In a mad dash / dive I pulled the drain valve and saved the day. Hadn't done that again since.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.