started offering a second 12 gallon fresh water tank recently. The grey one is 26 gallons, the black 9. You can find this info on their web page under the "Ask Scamp" tab, then Maintenance, External. It really should be easier to find.
I always have a collapsible fresh water bladder, almost 7 gallons, in order to have water while traveling and to bring water to the campsite. I also have a five gallon plastic bucket with a lid, so I can haul grey water to the dish washing sinks or toilets at the campsites, if necessary. It could be used for the black water as well, but only when really, really necessary.
For travel, I always dump all tanks to keep the camper light
and keep the full water bladder in the bed of the truck, in front of the hitch, for day use.
Almost exactly a year ago I found a 2004 5th wheel deluxe and like it very much. I tow it with a 2013 Tacoma 6 cylinder, already put about 7k miles on it last year, and I would recommend it. I get decent mileage, ~17 mpg, and it tows beautifully.