[JRTJH]

The 2018 Passport brochure advertises R-11 roof insulation.

If you compare to "house insulation", roll batting designed for 2x4 walls (3.5" thick) is R-13. Typically, RV companies "maximize R- values" by adding the insulation value of the OSB decking, TPO sheeting, luan ceiling panels together to get the "best advertising boasts" as possible. So, I'd suspect that the insulation in your roof is probably 2.5" - 3" at best.

Can you blow in insulation? I'd ask how you're going to access the areas to get "blown in insulation" evenly distributed and whether adding it would be worth the effort when you've still got R-7 walls, single pane windows and "holes everywhere that leak heat/cool air from the cabin.....

As Chuck said, you can pretty much "do anything that fits" to an RV, the question is whether it's wasted effort or a means to improve some aspect of the trailer. "super-insulating the ceiling" likely falls in the "wasted effort" category when you consider the rest of the "energy inefficiency" that's left after all the effort..... YMMV