On our '05 Itasca Sunrise the reserve tank is right below and back of the driver side entrance door to the cab. It can be accessed by opening the outside basement storage door closest to the driver's door. Our's has a cap on the tank that also has a fluid level stick built into it. Our manual says to use Dextron 3 hydraulic fluid when refilling it, something I plan on doing next week when we take it out of storage. Hope this helps, not same model but it is on a Workhorse chassis.
Steve & Nancy
2005 Itasca Sunrise 33', W20 Chassis, Ultrapower, Henderson Trac Bar
2012 Chevy Captiva Sport AWD, ReadyBrute Elite Tow Bar, Blue Ox Base Plate, Protect-A-Tow