Yep...if you have a P-30 chassis, at least on my 77 Itasca, there is an oval trap door hidden under the carpet secured with four countersunk phillips head machine screws. Sitting in the driver's seat, looking down at the floor, it is just to the right of the steering column. When I recarpeted the cockpit and found it, I made sure I cut around it, and then carpeted the trap door so I could check my fluid with ease.
Now, I'm not sure if it was a Chevy thing, or if Winnebago made the floor and thus the trap door.
Also accessable through that trap door is the control center for the cruise control, which I have yet to make work!
The master cylinder sits forward of the opening a bit, but you can still easily remove the bales to remove the top, and look right down inside it with ease...use a small bottle of brake fluid to top it off...there is room for a small bottle without spilling it, but you'll never get a big bottle in there.