3yrs ago I bought that -Bubble- insulation at Walmart in Murray,Ky in the weather strip dept. Also you might use that painters removable tape and tape the bubble wrap inside the boat , or Velcro in the sewing dept,(as I did in 9 degree weather in Kenlake marina).
.....For me, the Tarp was helpful, but ugly. Of course the inside bubble stuff was ugly. But you have even more cold than I did. I also used a Mr. Heater at times for a boost while awake with No carbon Monoxide readings. You could also plug in -two- shorepower
connections and buy another heater. **I did all of the above and shut the door to the front cabin & slept in the main cabin. (as you & wife know that humid cold is miserable)
I wasn't sure whether to put the bubble wrap inside or out, so I did a mixture of both to see what worked best. Yeah, I know... Duct Tape.. But it is on the metal and not the hull and I have goo b gone stuff that is supposed to clean off the mess it's bound to make.
Yes, we have a Mr. Heater too and have never run it overnight due to the fear of waking up dead. They get expensive to run over time and opted for a radiant electric heater that works well on low.