Brake problem

I have driven in mountains for years, I know not to ride the brakes and to shift down. I was not in heavy mountains for the last 50 miles at least and was on a slow road with a speed limit of from 35 to 55 mph. The brakes worked fine when I got to the campground. I left the engine running while I went and signed in, that took 5 or 10 minutes and then I slowly drove to my camp site, that is when I noticed the pedal sinking to the floor. I think I will go to the Chevy garage in the morning and make an appointment to have the fluid changed and if they think the master cylinder is bad, I will have them change it. Although I agree the fluid should be changed I really don't think the brakes were that hot. I just had the MH in to the Chevy garage back home before the trip and had them pack the wheel bearings and check it over including the brake pads and all fluid levels to the tune of $830 so it is rather disappointing that we are having this problem. Oh well such is life, I am just happy it happened at 5 mph and not 60 mph. Thanks again for your ideas and opinions.