Definitely keep the roof vents closed.
The portable AC units probably won't help. At least the ones in the price range you are talking about won't help. They will exhaust out the window and then pull fresh hot air in from outside to replace the air that was vented out.
Do pull the center vent down on the interior of the coach and inspect what's up there. The AC is a sealed unit meaning it does not pull air in from the outside. Instead it pulls cooled interior in and cools it more and then pushes it out through the vents. There should be a divider up in the ceiling between the air coming in and the air going out. Make sure that divider is standing vertical in the opening and sealed with some duct tape to prevent air leaks between the two halves.
Buy a roll of good HVAC tape to seal inside the AC box in the ceiling. Do not use the cheap $1 duct tape. You want something like this.
HVAC tape
