I didn't have a shower/toilet, but I used one of these for 20 years with my pop up (the 5 gal model). It fit neatly under my dinette table on the road (make sure you keep the outside clean!) It came with a short (~ 2ft) piece of standard hose with a fitting on each end. One end connects to the tank cover, one end connects to the outlet from the sink.
10887 Barker 5 Gallon RV Tote-Along Portable Holding Tank
When it gets full, take it to the CG bathroom and pour it down a toilet. Very simple. With a shower, you probably would need a larger one, in which case, you'd have to dump it at the dump station. The bigger ones let you attach them to your TV's trailer hitch and pull them. Only issue with the bigger ones is where to store it on the road.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10