I would definitely mount a support plate under the slide floor as well, if there is room. It would work on our camper's slide.
The under-floor support plate could be bolted to a base plate above the floor, making sure to cover two floor "joists" with the bottom plate, using carriage bolts with the nuts on top. That would minimize the interference with the slide seal.
Then the telescopc base could be attached to the base plate. That would result in a very strong table, and no worries about damaging the floor from someone pushing down on one side of the table.
A bit more work, but definitely a superior result.
Just my 3 cents!