I have encountered this problem a couple of times. If you want to try to fix it yourself without taking it to the shop you can try the following:
...The cable that the parking brake runs in is probably corroded. Buy a good quality penetrating fluid and spray the enclosure that the parking brake cable runs in. I have found that WD 40 works pretty well even though it is not technically penetrating fluid. Leave the soaked cable sit for at least one hour. Thereafter go under the rv and move (bend) the flexible enclosure that houses the cable, frequently this will break this cable loose.
...Sometimes the cable seizes up where it goes into the "backing plates" at the rear wheels. Secure a small hammer and tap moderately where the cable goes into the backing plate. spraying this juncture with penetrating fluid may help loosen up this location also.
I have an older rv and find that getting the emergency brake to work can be problematic if I leave it set with the brake on. I would recommend that you securely block the wheels and forgo engaging the emergency brake during storage periods. If you have placed your transmission in "Park" your rv should not be able to move anyway.
Good Luck. Dennis