In my mind the most important option you can get with a Four Wheel Camper shell is the furnace, unless maybe if you travel and camp strictly in the summer or in the deep south and at low elevation.

We know some people use portable propane heaters but we did not want to go that way for several reasons. They have been found by some to be reasonably safe with some degree of ventilation, but how much ventilation is not clear, and ventilation means more cold air. Also, another combustion product of portable heaters is water vapor (propane + oxygen --> CO2 + CO + water vapor). Because we expel significant water vapor when we sleep, there is already a condensation problem on cold nights so adding more water vapor is not something I wanted to do. I prefer my heater to expel combustion air outside the camper for all these reasons. And the Dometic Furnace that came (as an option) in our Hawk Shell works great!

Another reason to get the furnace option with your shell is that is comes with the propance compartment and plumbing all the way to the furnace. The sweet thing about this is that FWC puts a 2-port propane manifold in the furnace box. The second port is plugged, but it is located just below the furnace compartment countertop storage bin so it is very simple to attach a second propane hose to feed a stove or burner located on the countertop above the furnace. The furnace and our 3,700 Btu/hr burner both run at the same time so there is plenty of capacity in the manifold.

We live in a cooler climate and try to camp every month of the year. For that reason, we purchased the Thermal Pack. It is clear that it helps insulate in cold conditions and on hot days. It also separates the living area from the outer flexible wall which can become wet from condensation on very cold nights, due to the moisture in our breath. Perhaps if we leave the furnace on all night condensation would be reduced, but we don't do that.

We opted for two Fantastic Fans to make sure there is plenty of ventilation. One is over the bed for cooling and the other is over the "kitchen"/"bathroom"/living area to exhaust unwanted odors or steam and to aid in cooling. You can set one to blow in and the other to blow out.

FWC now offers another style of fan called Maxx, I believe, which people seem to prefer so be sure to find out about that if you are making a purchase.