Foldbak,
I know you have been camping for some time and you are very familiar with your camper. By now, you should have an idea how often you (plural) need to dump the tank. But, here's the catch.
If you have had full hook-ups, you are probably not aware of how much "stuff" actually goes into your tanks. So, what you need to do is monitor how much is going into the tanks (or any one tank), so when you dump, you do not overfill the portable sewer tote.
Nothing is more aggravating than overflowing the sewer tote and having a mess on the ground and also a mess all over your tote.
I would suggest, if you've never really monitored your water usage vs how much is in each tank, then initially you go no longer than 2 days before your first dump. If the tote still has capacity, then dump the next tank, and it still has capacity, dump the next tank (If you have 3 tanks).
Now, the next time, go 3 days. Again, dump only one tank (the black tank) and determine how much it fills the tote. Again, you don't want it to overfill the tote. If it does, you know you cannot go 3 days. If it does not, you know you are safe for 3 days.
If still safe, now dump one of the grey tanks. Again, if it does not over spill, then double check the amount inside the tank, and then make a deterrmination if you can the third without overfilling.
By doing this, you will eventually learn just how long you can go between dumps. Then you need to determine if you want to get them on a cycle so they fill up at different times and you end up dumping only 1 of the tanks at a time, rotating them on different days.
It really depends upon your usage and how large your sewer tote is. You'll need to experiment a bit and figure it out.
For us, we have 3 tanks, 2 grey and one black. We very seldom have full hook-ups, usually just electricity. So, I depend on my Blue Tote for dumping my tanks. I have learned we can go 2 days, and I can comfortably dump all 3 tanks without overfilling my 39 gallon Barker Blue tote. If I go 3 days, I usually overspill. So, if going 3 days, I'll dump them one at a time, and if necessary, make 3 trips to the dump station.
Now, I also back flush my bathroom grey tank into my black tank. So, I really need water in the grey tank to do so. Doing this, I never have to have a full black tank to dump. The backflush cleans things out, much better than just a single dump with an absolutely full black tank. And that is another reason why I dump ever 2 days, because I backflush.
When backflushing, half of the grey tank will back flush into the black tank. Close off the grey tank and now dump the black. Again, watch the level in your sewer tote. If all is good, then back flush a second time. Now it will only be half of the original half. Each back flush becomes half of what was in the grey tank. And each dump the water in the tote raises slower and slower, until you reach the top. When that happens, you have a cleaned out black tank, and the grey tank is now almost empty.
Of course, in order to back flush, you need that 3rd valve on the end of your discharge port where the black and grey tanks come together. But doing so will really clean out the black tank without have to wait until its 100% full and holds a lot more contents than your tote does.
Experiment with 2 days first, then 3, and so on, until you get a feel for your usage, how much will hold in the tote, and do so without making any spills.
You'll get there. You CAN do it!
You have way too much time on your hands lol. 
