And how long do you want to save your poo? Are you that attached to it?
Seriously, how often do you plan to dump? We have (currently) an 18 gal waste tank in the RV with 3 adults using it. That is approximately a dump every 2 or 3 days. We don't have to move the RV since we are on full hook-ups. The bus' black tank is roughly a 25 gallon tank but will have the bathroom sink plumbed into the black tank. Most problems with black tanks comes from not enough water. Also we couldn't see running another grey tank just for the bathroom sink nor running a drain to the other side of the bus to the grey tanks. I'm estimating on our black tanks size as its a used poly tank from an older RV (but in good shape) and the form is a tad odd (compartment will be sized to hold a rectangular replacement ABS tank because it's only a matter of time before the poly cracks). My "blue tote" only holds 30 gallons. I don't want to overflow my tote with ANY waste water. A 25 gal black tank should be good for a week with 2 adults as RV toilets only use about 2 cups water (pee) to 8 cups (poo) to fill bowl and flush (and that's not being very conservative with the water). After that we would have to dump. Means if no sewer hook-up, we either pull out the trusty blue tote (two wheeled, gets very heavy, very quickly... best to have a 4 wheeled side discharge tote if you are in the market for a tote) and haul it to the dump station behind the jeep or we would have to move the bus. Our bus will be for full-time use... just like the RV currently is. If you will be boondocking, there are septic tank trucks that can and will pump your tanks out... for a fee, just like they pump out the port-o-lets/porta-johns on job construction sites (same folks). I have even seen them in state parks, pumping out a big RV with big holding tanks (will take both grey and black waste water). Moving, once set up, just to dump tanks is a PITA. Been there, done that, dug my blue tote out of the storage unit and took back with me (a 300 mile trip one way "home" to fix something for an idiot with more $$ than brights) so that I wouldn't have to do it again.
(Okay, now I am going to end up going off topic but I have a thing about the tankage situation...)
Only thing about black (and grey) waste tanks is that they cannot leak. Not accidentally or deliberately. But you may want to get a standard tank that you can easily replace. Plus once you get past 15 to 20 gallons, you will want to start thinking "baffles" to break the water movement. Baffles in a black tank is a nasty thing as "stuff" hangs up on them. Just deal with it. The only time you will look into your black tank is if you drop something in it or if you get the dreaded "PILE OF DOOM"!!! (solution... 5 to 10 gallons water, a 10 lb bag of ice plus a ride down the road for a few miles = a clean tank)
Get an ABS tank so that you can repair it if (and when) it cracks. The poly tanks are a PITA to repair unless you are proficient at plastic welding. ABS can be repaired using standard "all-purpose pipe cement" found in any plumbing section of just about anywhere. You can also cut holes and install fittings that you choose (like big clean out access ports, tank sprayers, etc). If we had not already acquired the poly tank really, really cheap, we would have gone with an ABS Valterrra Rocket tank (via JC Whitney or whomever is the cheapest price including shipping costs) and simply stacked two together with two or three of the biggest glue up pipe connections we could find. Home Depot and Lowes has a nice selection of ABS drain pipe fittings in their plumbing section. I have hole saw sets that cuts a neat round hole up to 4" thru most anything. I've noticed (since I'm usually the one who usually dumps the tanks) that everything in a black breaks down pretty well over two or three days without any added chemicals. So the idea of stacking two Rocket tanks on top of each other with a few big pipe connectors glued between the tanks (number depending on length of tank) seems feasible to me. Plus there's the added bonus that if the tank develops a crack, we can easily patch it... unlike the current poly tanks in the RV... which I have to drop and replace (with rocket tanks) once the bus is finished.
handy little things. Comes in a standardized height and width... only the length changes. We will be using a couple more of these as I have a 30 gallon Rocket tank in storage already (total of two 30 gallon tanks for 60 gallons fresh water plus two more 21 gallon tanks to match the other poly tank I have set aside for 63 gallons grey water, although I may flip that since the fill valve won't allow the tanks to fill to the top). The fresh tanks will have shut off valves so that we need only travel with 1 or 2 operational tanks if we want. But we will be filtering (0.5 micron) the crap out of our water which drops the water pressure really low plus adding a water softener before the water ever get to the holding tanks, so we will add a shut off float to one of the fresh water tanks so the filtered water will fill the tank(s) until the float determines the "full" point and shuts the water fill off. The fill valves are brass with plastic floats and are used for swamp coolers... something I would have never though of if we hadn't come out here to NM and seen them in the local ACE hardware store.
I have put some thought into the tankage and I am just a tad paranoid about water quality. We've been in campgrounds with seriously bad water(I blame the early demise of one of my cats on that campground's water). I have also been fighting with those crappy POS poly tanks for a few years now. The patching/repair compounds which are "supposed" to work, don't last thru the heat/cold weather cycles all that great. And the poly tanks have to be drained, dropped, DRIED completely before patching. Last time I dropped the grey tank, we were tankless for 4 days from removal to re-install. With an ABS tank, it would have only taken a few hours before the patches were set and the tank re-installed. And they make pipe cement in "wet" formulas too, so "perfectly dry" isn't really a problem either.