WE FTd in a 34' 5th wheel for 7 yrs.
Roughly 280 sq/ft
40' is only going to get you and extra 40 sq/ft (320 sq/ft)
But is going to limit some places due to length
Unless you plan on new truck keep what you have and go.
Mobile Suites....not enough truck plain & simple
As for 'weather'..if your current 5vr does not have an enclosed belly.....slap some insulation up in there, run a furnace duct down into area around tanks and enclose it with sheets of coroplast (sign shops/hardware box stores and self tapping screws)
Besides..when FTng.........follow the SUN