There are several types of TPO...
My TPO roof doesn't have a backing... it is thick, white on top and black on the backside. It is held around the edges and by the vents, A/C etc. I think that self leveling Parbond can ce used at joints like between the front of the cabover and the roof. I'll confirm my parbond statement later if you need it.
Bob Mehrer at Snowriver used a TPO with a fuzzy felt on the back... they glued it down. Makes a very nice job... I watched them install a roof on a new camper. His crew even took a router and rooled the edges of the roof befor installing the TPO
Lance was using the Glue down type the last time I was at the factory.
The glue down type has another advantage... it is lighter in weight and the TPO cross link structure strengthens the roofs structure when the camper is trying to twist and flexing.
I'd like to have my whole camper made out of carbon fiber or TPO