For me, the windows are the biggest exchange of heat and cold. It's no fun, but we put Reflectix on most windows.
I have put Reflectix down on the one bunk that is above the outdoor kitchen, but I've been thinking of replacing that with 1/2" foam insulation.
I also support and tie down my awning (thanks to FRF member, OldCoot) so that I have shade all the time on that side.
I try to have tree coverage for shade.
I know that another member here insulted the front of his fifth wheel. He pulled the shoe box and insulated behind of it.
But you're right- these rigs aren't well insulated. The one that I've been debating "how to handle" is the floor. There's barely any insulation under here- just minimally around the tanks. I've debated pulling the undercover down and seeing about some kind of insulation plans.