Tin foil is a quick room darkner, and something everyone has in their RV's kitchen.
I prefer the black Maxxair vent covers over the bedroom. The fresh air flow is nice so I like the vent open, and the black covers help block the light.
Both our RVs (and our cabin bedroom) are outfitted with blackout curtains in the sleeping areas, which means I can also open the windows and still have a dark room. I purchased the blackout curtains at Walmart then cut them to fit the RV windows.
There are also blackout roller blinds that are very effective. I use one of these on the RV door window. Rather than attach it, I cut the roller off and then just drape the vinyl material over the screen and let it hang over the door window. Then roll it up in the morning and tuck it in a drawer.
If you go this route, make sure you get the 100% light blocking items.