I have the same problem with my 2017 17QB, also my first camper. Holding tank size was not among my considerations when we bought it. And now I'm finding them, at 9 and 15 gal, to be ridiculously small. And I'm a little jealous of the bigger rigs, and moreso the similarly sized rigs that I know have larger tanks. With this camper, as is, I guess the expectation is to buy a tote and use it almost daily, or stick to short trips, even still using your facilities sparingly. Not really what I was going for.
On the plus side, I can dump both of the tanks when full into a manageable 25 gal tote. But I really don't like the idea of driving all the way through the campground dragging a barrel of poop, so I strain to heave it into the truck. Actually, I back the truck up to the picnic table, hoist the tote onto the seat, then onto the tabletop, and then pretty much roll it into the truck. It's cumbersome indeed, and I'm probably due for a disaster. The other problem with this is that because of the low ground clearance, the outlet is usually just a little lower than I need it to be, and I have to work the hose up/down to manually pump the fluids into the tote.
The first solution I considered is just what OP asked about. Looking underneath, it seems to me that it would be *relatively* simple to relocate the gray tank to the black tank's position (adding 6 gal capacity) and replace it with a 25 gal tank that I had found with the same dimensions, only wider (adding 10 gal capacity). I too wonder just how difficult this would be. I had seen some rubber grommets for sale that are used for connecting the pipes to the top of the tank (such that they slide in/out). I wonder if these are indeed what's in use here. I suppose I could ask SC, but I have not. If that's what it is, then I should be able to drop the tanks just by dismantling the external plumbing and unscrewing the brackets. Quite possibly not all that difficult. But the drawback to this is that even after all that work, I may still need to use the tote (and would need a larger one to do both tanks at once), and have to do the manual pumping and certainly the dragging of the barrel of poop through the campground.
My other solution, which I have yet to implement, is to leave the trailer as is, and use a macerator pump with an auxiliary tank in the truck. Seeking the least expensive option, I found this $62 macerator (
http://a.co/9QEUgK5), and this $120 44.5 gal holding tank (
http://a.co/2uU7XRe;
http://www.alphasystemsinc.com/files/ca ... 5-2012.pdf). The raw tank will of course need some fittings ($), and I'll build a little frame around it so it can sit functionally in the truck bed. With this setup, I should be able to easily transfer my full tanks to the auxiliary twice before having to go to the dump site. Then I just drive over and hook up, same as I would normally instead of fumbling with the tote. Basically, I'll have my own personal honeywagon.
I've been looking around online for someone mentioning using an auxiliary holding tank in this fashion, but haven't seen anything. Has anyone ever heard of a setup like this?