What defines a TT as a "True Four Season"?

I said all this because there seems to be almost a singular focus on how well an handles the cold, and no discussion about how it handles the heat. I think this is because there is a general consensus that, if a RV designed to handle the cold well, it will also be good for the heat. While this is true to some extent, I think the complete ignorance of radiant heat, which is he main source of heat in the summer on RVs, has lead to a misunderstanding that a good cold weather RV is best for summer heat as well, and this is simply not the case.

Considering all the above information, my question then is. Are there Travel Trailers that are specifically design with a focus on use in a hot climate where there will a lot more radiant heat then conductive heat to deal with?

If you are a builder, you should know that R-factor is a resistance to the transfer of heat and cold.
One large problem with RV's is that the Mfg's skew the R- ratings by tossing in a foil bubble wrap and add the large rating number into the mix. Since the bubble wrap is never installed to meet the high rating, i.e. installed in a cavity with no convective loss, it is a marketing gimmick.

What I did with my trailer is probably not what you want to do but maybe it's insightful: I gutted the whole thing from the inside. I packed in as much extruded insulation board as I could friction fit in the cavities. From the wall cavities, I then installed a 4 mil vapor barrier. Inboard of that vapor barrier is 1/4" fanfold extruded insulation. This latter layer removes any conductive transfer through the wall studs/exterior skin. Paneling was laid across the frame and secured with finishing nails and screws. I pretty much did the whole trailer like this--wheel wells, ceiling, floor.
I also eliminated a good bit of window glass and replaced what glass is left with Low-E sliders or a stationary panel. The roof was painted with a white acrylic coating. I replaced the RV furnace with a non-electric propane heater, surrounded with masonry. The furnace is piped for combustion and make-up air in the stack.

The result of all my work is a trailer that is easy to heat and cool. If I leave windows open to catch 70f night air and close everything up in anticipation of a 90f day, the interior temps at noon will be 75f.
I haven't done a lot of Winter camping but if it is like previous trailers I've upgraded, parking in average MN Winter temps of 10f, will mean 30 lbs. of propane/wk for heating. Water heating and cooking, which vary by number of occupants (as do door openings), will add to the propane bill, maybe an additional 20 lbs/wk.

You are probably not looking for all the work of a gut out and repack. Just try to find a trailer with solid insulation, thermal windows, and an efficient air conditioner/heat pump. Using some type of shade cloth in Summer and skirting in the Winter will add a lot of comfortability.