Great idea! I trust that is transition cement between the ABS & PVC? Been thinking of doing similar. We have one outlet at the rear for the kitchen and the other is under one end of the slide ahead of the axles and is hard to reach with the slide out. Very annoying! Problem will be getting adequate slope and getting over the axles but am planning to take down the underbelly material which might help a lot.
ABS is readily available in Canada as it's required by code inside buildings. Should be available at plumbing wholesalers in the US? Of course you could always go to an RV dealer and pay 10 times more for it. Had trouble finding some basic PEX fittings at retail stores in the US last summer.
Why do the factories do this stuff? Pffft...