I purchased a 25 gal tote n stor 4 wheel model.
I bring it with us if we don't have full hookups which is often, and only if we are camping for more than 3 days.
Like others, I'll never utilize it for the black tank.
I considered the 15 gal vs the 25 gal, but the price difference wasn't but a couple of dollars difference.
Sometimes I think it would be better to have the 15 gal since I throw it in the back of the truck when not in use, but I can empty the gray a good bit in one trip.
I'd suggest something - but that's just me.
2013 Rockwood Roo 23ss
2012 F150 Ecoboost
Camped for 12 nights in 2015 - too many cancellations
Camped for 25 nights in 2014!
Camped for 24 nights in 2013!