How much black water there is per week depends on how much you use to flush with. The human contribution is the lesser amount in the tank for most RV'ers with flush water being the greater. That said, you do need to use solids breakdown chemicals and enough flush water to breakdown the solids and limit the solids buildup under where the toilet dumps into the black tank.
I can only speak for us two as we generally put about 10 - 12 gallons into our black tank per week on average as we are careful. The black tank in our 29' Carriage 5th wheel is 40 gallons so it usually would last an honest 3 weeks before needing dumping. Longer than how long the gray tank will last before dumping and why we made some very simple cheap changes for our waste water system tank usage.
Removed the RV toilet (2 nuts and unscrew the water line at the joint and plug the RV supply line so water won't run out) and made a 5" high riser platform for where it used to sit out of 3/4" plywood with 2 X 6 cut down frame under it and a good looking 1 X 1-1/4" border around the top side nailed and glued. Sanded smooth, nail holes filled and painted to match the floor so it looks like it's OEM.
Had cut about a 7" hole in the middle of the top of the riser for access before assembly. Placed a plastic cap over the RV sewer opening and Gorilla taped it down so no liquids or smell could escape. Used the existing RV toilet threaded studs thru "L" brackets over the studs and the brass nuts on those studs to secure the riser in place.
We use a 5 gal tall off the shelf Thedford Porta Potti that sits on and confined by the riser top side border for our toilet and it's perfect as it has a swing out dump spout on the detachable bottom half waste tank for dumping in any toilet or even a pit toilet etc anywhere. No bad smells as you use chemicals which neutralize and breakdown the solids and NEVER need gloves. Even fuel stations as we have never have we been refused yet ever and never have spilled a drop with the spout or get any on your hands. So much easier and cleaner that dumping the black tank at a dump station that so many mess up.
Now, we have about double the gray tank capacity by simply adding an off the shelf RV gate valve which twists on just like the sewer dump hose does and the sewer dump hose twists on the other side of the added valve for dumping the now 2 tanks as each has it's own tank sliding gate valve. Leave both the tanks valves open and you now have double the gray water capacity. If needed, in an emergency, can go out and close the prior black tank valve and dump the Porta Potti in the floor black waste hole and it's just like when it was first built.
You have added no holes in your RV nor defaced anything and in minutes can change the whole system back to OEM for resale. The top flush tank holds about 3 gallons so you'll know how much water you have added by how often you need to refill it. WE need to about every 10 days for just the 2 of us.
Another huge advantage is that we do the very same thing on our Lance TC as we use the same Porta Potti it in the winter time too when it's freezing outside and the TC sits with no heat on when parked at home. Simply by using buy anywhere cheap off the shelf standard vehicle windshield washer fluid that has alcohol in it for freeze protection in the top flush tank prevents any freezing of the waste contents. All those windshield gallon plastic bottles have the freeze protection level on the label. Even if it could freeze, it would only be a slush. Been in the U.P. of Michigan and up in Ontario Canada in the winter in below zero temps and never had any ice in the waste tank and the heat wasn't on in the TC while we were on a 3 day train tour. Never at home in Michigan either.
Toilet is always ready to use anytime anywhere too and no matter the temp. Cold seat? No problem as you sit simply on the seat with your pants on for a couple minutes which warms it before dropping your drawers and getting serious! Worried about the toilet tipping over when traveling on a bad road? Simply put a adequate screw eye in the rear of the riser and either use a Velcro strap thru the PP handle and the screw eye to prevent tipping or any other type hold in place strap or rope etc as most any method will work and remember that the riser is bolted down!
Just some ideas for real versatility for you...
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT
