I used one of these 5 gal ones for 20 years on my Coleman Sequoia popup.
Barker Tanks 5GALTT
10 gal would have been nicer (dump fewer times), but storage would be more difficult.
Keep it clean and store it right under the dinette table while you travel. You can dump it straight down the toilet if the bathhouse/outhouse is closer than the dump station. (Same with your porta-potty.)
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