While camping at state parks that do not have a sewer connection at each site we found the black tank can go over a week so one trip to the dump station was not a problem in a two week stay. Both of us using the shower every day used up the gray in three days or less even when trying to conserve. Looked at the "blue thunder" tanks on wheels but really didn't have a good place to carry one. After talking to a neighbor at a CG that had a bladder or pillow tank I looked into them. Not many sources but I found a 25 gallon one here Water Storage Tanks | Tanks for Fuel and Water Storage
They were very helpful and followed up after the sale.
The top of the tank has a 1 1/2" opening and the side a 3/4" threaded. For the top I used a 1 1/2" to 3/4" pipe thread PVC coupler then a 3/4" pipe thread to garden hose nipple. The side opening just needed the nipple. 90° RV hose adapters worked well to turn the connections. An old garden hose was used to change the gender so the female end could connect to our macerator. Shut offs are on almost every connection for obvious reasons.
At the dump station I use a utility pump from Harbor freight and short piece of garden hose. A jump start box works well to power both pumps.
At 25 gallons it holds about half the capacity of the tank so a couple of trips a week does it. I'm not sure if I have the courage to try it with the black tank