Help with floor ducted furnace, lack of air flow

Don't you just pull up your floor vents to get into the ducts? Mine are screwed down, but other than that they lift right up for cleaning. If that photo is similar to your RV, the one vent is near enough to the "T" to see in there. My endoscope is like a stiff wire, so can be bent and fed around corners. You may have to get one similar.