On my fifth wheel I liked to measure the floor and make sure is was level end to end. I do alike to drop the back side opposite the front doo down a tad to keep the roof run-off going to the back side rain gutter. I use a Carpenter long level...
On my OFF-ROAD POPUP I have to get the end to end very level otherwise I am not able to freely open the entrance door. Here again I always drop the back side opposite the front door to allow roof run-off to go down the back side. I installed a two-inch bubble level gauge near the frame right next to the door on my popup. This is not as dramatic as the fifth wheel as my POPUP trailer only has the built-in rain gutter on both tent bed ends...
Similar to this google image...

I also carry one of these Stanley magnetic short levels in the Rv TOOL box which is handy to have around as well...

In both trailers I like to have the roof tilted away from the front awning...
Roy Ken