Since it is an added item from other dealers, it is not shown on the normal Winnebago drawings but should be quite easy to spot by looking underneath.
It is often at the edge of the frame and somewhat close behind the front wheels. Maybe different but often that is where I would expect to find it.
And since it is a large item at near a foot or so and with hydraulic lines running from it to each jack, not hard to find. Worst case, look under the frame, spot a hydraulic line and follow it to where they all come together.
There may also be an access door to open from the outside. Some do, some not but if you have an ouside door you have never looked in, that might be the good place to look!
Small point to watch for is the way there may be two different opening/plugs that look like a place to add fluid. If you find one on the side, ignore it and look on top of the tank as opening the side will often get you covered in fluid, also the level there will be way off from what is needed!
One type for example of what to find: