Portable Holding tanks

If you add a portable holding tank do you have a way to empty that tank? Or are you considering just extending the time between pump outs?

As said above do not leave the black tank open unless you want a huge pain in the a-- when (not if) the tanks plugs up. Whether you are using the factory tanks or an added on dump tank leave the factory dump valve closed until black is at least 2/3 full. 

It's okay to leave the gray valve open the whole time. Though many people leave it closed as well and use the contents of the gray to wash all the black contents out of the dump hose.

And don’t just leave your black tank open. Great way to fill/plug it up. Run it closed until almost full and then dump. Google poop pyramid. 

Totally plausible solution. With some planning and caveats. 
Like making sure you can pump out the holding tank sufficiently enough to get rid of solid waste. 
Another consideration is what you mean by “property”. I’d find any reasonable means possible to dump gray water back into the ground vs paying to get it pumped regularly and worrying about conserving every drop of water you use in the kitchen and shower. 
Even though there are plenty of people who don’t understand and just say it’s “bad” to dump gray water out, that’s the primary reason the 2 used water sources have been separate in RVs….forever. 
If you can’t or won’t dump gray water on the ground or makeshift pit, etc. then yes your idea will save $ in trip fees for the turd burglar.