Four Season Truck Campers with a Slide- really four season?

I bought my Fleetwood for the slide, but it come with basement and attic.
On my way from Minnesota to California I slept in it at about 8F and was comfortable.
But I put the slide in and sleeping on the dinette sofa, put a curtain at the upper bed.
Doing some dryrot repairs I discover lot of holes in the camper that would allow heat, or cool to escape and I sealed lot of factory holes, so I am pretty confident the camper will withstand extreme weather.
Either 0 or 120 F