You need to go to lots of RV shows and look at the various models and floorplans. In our case we first purchased an old non-slide trailer. This allowed us to figure out what we wanted in a floorplan.
For example, when we finally started looking we found the following features were a minimum requirement for us most involved the slide.
1) Access to toilet with slide in.
2) Access to bedroom with slide in.
3) Access to refridge with slide in.
4) Access to pantry/cabinets with slide in.
5) Rear living layout.
6) Free standing table and chairs.
7) Large water tanks for extended boondocking.
There are probably a few others that I have forgotten about.
In the end we purchased the Jayco that is in our signature, which met every requirement.
Just the other day we were out looking at trailers while picking up some parts at the dealer. We are still happy with our layout and did not find any of the new trailers that would work better for us.