On the Jayco Eagles the only difference is that the Thermal package provides higher insulation values on the walls, roof, and front cap, and has a slightly larger heater. Both models have the heated and enclosed underbelly.
Mine does not have the package because I bought a year old model off the lot at a steal of a price and it didnt have it equipped. That said - my sons race on a ski team and we camp every weekend up in the mountains all winter. Temps drop below the 20s frequently and we have never had a problem staying warm (condensation is another issue...but the thermal package wont help you there).
BUT....
If I had the choice I would have bought it - because - it is only a couple hundred dollars and you only buy your TT once....so....get the features just in case.
But....
Do not worry yourself if you end up with the model without the package, because, as i have stated - you do not need it to camp even in the dead of winter.