Top access to the engine will vary widely depending on the model, year, and especially the floor plan. So unless you get an answer from someone with the exact same model, year, and floorplan, whatever answer you get will just be generalities.
On my '07 Endeavor, the bed is on the side slide, so the engine hatch is easily accessible on the floor in front of the bedroom closet -- just push a few latches and lift it up.
So, as you can see from the wide range of answers here, there is no one size fits all answer. You're really going to have to get the seller to show it to you on that particular coach.
Adam and Sue, and a pack of little furballs
2007 Holiday Rambler Endeavor 40PDQ Limited Edition - Cummins ISL 400
2013 Ford F-150 FX4 toad - USGear Unified Tow Brake, Roadmaster Blackhawk II Tow bar, Blue Ox baseplate
Home base near Buffalo NY, often on the road to a dog show