Even with the keel tube inflated the flexible floor fabric deforms/bends where the bunk rails are and contacts the floor, but I deflate the keel tube on any long haul so the material isn't being stretched tight and the boat sits firmly against the bunk rails when strapped down.
Supporting the transom one way or another is definitely important.
The two small support pads up front really keep the boat from bouncing up and down on rough dirt roads.
When I reconditioned my trailer I replaced all of the hardware like the u bolts holding the axle assembly with stainless steel. I also reinforced the the joint where the trailer sections join with a cross piece. The first photo (of a floor repair) also shows the two vertical bow supports. The second image is of the added frame reinforcement. Lots of trailers and boats on the side of the road in Mexico! I'm glad I took the time to get the trailer right before I started bounding over dirt roads.
Anyone towing in dirty gritty conditions should have a full cover and chafing protection on all tie down lines.
The little black patches visible in the first picture cover wear spots from the previous owners repair attempts, those patches are not there to hold the floor to the tubes.