My brakes on my coach work OK but not great, 4 wheel disk brakes, If I stand on them it stops , before this coach we had a1980 Prevost with a tag, I know it was drums and 2 more brakes, But on Foretravel I would think they would be more sensitive, specially in reverse and cold they just don't seem to grab to well. I was on a steep hill facing up hill with tow car attached and my wife was checking in and spring brakes were set and it creep-ed back a little so i had to hold breaks on, freaked me out a little, It was a pretty steep hill. I had rear rotors and seals replaced 4 years ago because of a seal leak, pads looked OK , maybe pads are hard and just don't grab as good as they should . I don't know if they were ever replaced, Is there different pads hard or soft?
It would be a good idea to have your springs in the park brake chambers checked. They do break, you might have only one holding. Also something might have happened to a automatic slack adjuster. If in doubt throw wheel chocks under it for safety.
I have the same issue with the 1996 Monaco Sig. Same Meritor disc brakes as Foretravel. Cold, with spring brakes on it will creep in reverse, once they are warmed up problem goes away. Haven't noticed it in the '99. Drum brakes fade when hot and discs get better.
Could be your pads are glazed a bit. Some times if the retarder is used all the time the brake pads never get hot enough to work properly. As to your parking brake the slack adjusters could be out of adjustment. They should have no more than a 11/2" or so of free travel. They are self adjusting but still need to be visually checked once in a while. I don't know of any pads other than Meritior