I know I'm asking this 5th wheel to do what it was never designed to do, but I'm in it now.
IMO you answered your own question.
Your not going to "stuff" more insulation under the shell as you call it. It is what it is.
The wife and I lived in our 27' 5er one winter. Condensation was a huge issue in the areas you mention. Our condensation didn't freeze as zero temps here don't stay that cold for long periods.
We tried running 3 fans to move air around. Made a cold drafty camper and still had condensation problems.
We tried cracking vents and/or windows and boy did that ever create more cold drafts in 15-20 temps and 35-40 mph winds. That did nothing for our condensation.
I did skirt the unit with mobilehome skirting which helped with heating issues. I also made inside window covers from 1/8" Lexan which helped more than anything on keeping it warm in the trailer but still had severe sweating issues.
Finally bought two big dehumidifiers and within 3 days our sweating problems were gone.
Heating and cooking with LP creates moisture. I see where some folks switch heating and cooking over to natural gas when available, which they say helped a lot.
Many times a RV/mobilhome parts stores near you that deals with thin walls in a RV or older mobilehome and cold temps can help. Or other folks that use their RV as a home while working contract in that area may have some ideas for ya'. Good luck
"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers
'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides