Unfortunately, unless someone else has the exact same year, length and submodel, you'll never get a definitive answer. If the fresh tank is mounted inside, under a bench you can measure length, width and height. From this you can get the volume and calculate the capacity. You can pour 5 gallon pails of water down the sink and watch for it to bubble out the shower drain. You can do the same with the toilet.
Fran, Mary & Zoey (silver Cocker)
2006 Itasca Sunrise 35A, 2005 W-22, Allison 1000 5 speed
'13 Ford Focus ST or '10 Prius on Master Tow Dolly 80THD-SB or '00 Jeep Cherokee 4-down