Depends on slide out construction, but basically you have to find and use the hard points in the slide. These would include existing sofa attachment locations, seat belt anchor locations, third choice being location of any frame members in the slide floor or wall. I would be inclined to build adapter plates to bridge from any tie down points on the new sofa to the mounting location of the old sofa. Material and weight will depend on how well you want to anchor it.
Many folks don't bother to anchor replacement furniture. They may or may not bungee it in place for traveling.
Tom Test
Itasca Spirit 29B