We had a 310. Yes, the big windows make it harder to cool when the temps get over 100 degrees. I bought a big roll of the silver foil/cardboard stuff at Camping World. Then, I cut pieces to fit inside all the south and west facing windows. It made a huge difference in the cooling. I only had to run 2 A/C units, instead of all 3. Of course, the temps were over 100 every day for 3 weeks. It would probably help with heating too, but even though we used the RV in 20 and 30 degree weather, the heater and fireplace did fine.