"R" values are real world numbers, basically how much resistance to heat transfer a material has. The higher the R the slower the heat loss. A high R value is nice but only part of the story.
Max fill is an adverting term for something like "we crammed as much insulation into it as we could". Since TT walls are not all that thick and I doubt the TT folks are going to use a high dollar NASA type insulation, most run of the mill units will have about the same heat loss.
It is my understanding that the 4 season TT's do pay more attention to heat loss which often include double pane windows, attention to weather stripping, and potential cold spots; sometimes adding axillary heating.
My 2010 Dutchmen has fiberglass batting. How well distributed it is I do not know. The AC does keep it cool in 90 degree weather and we have camped when the temperature was in the high 20's at night with just an electric space heater. That is good enough for our needs.
