http://www.nadaguides.com/RVs/2008/Four-Winds/M-26BE-E445-V10/3049271/Specs
Don't know if yours is the ford or gm version but a quick look on nada guides for an '08 four winds siesta with ford chassis shows:
Holding Tanks
Fresh Water Capacity (gal / L): 33 / 124.9
Gray Water Capacity (gal / L): 33 / 124.9
Black Water Capacity (gal / L): 25 / 94.6
Propane Capacity (gal / lbs): 9.6 / 40.9
Water Heater Tank (gal / L): 6 / 22.7
2008 Winnebago Access 29T
Class C w/Ford Chassis