Replacing a couch should be a fairly simple project. Just remove the old one, (it is probably screwed down), and measure to be sure that the new one fits.
There are several ways to secure it to the floor. I replaced one with a sleeper sofa and used metal strapping and screws to secure it down to the floor of the slideout.
Make sure that it will fit thru the door also.
The couch does not have to be an RV couch, shop around.
If you are not comfortable doing this most any handyman could handle this project.
Good Luck and do not be afraid to tackle this task.