Any tips for heating/insulated bed with shell?

Hey there all!
Here's the deal.. I have a 2020 LB taco. Already have a Snugtop High Liner shell on it.
I plan on doing a lot of bucket-list camping this year. A lot of it in the South West. Think dessert.
These trips will be 1-2 months at a time between pit-stops at home base. Maybe more, who knows, I'm retired. I've got no where to go, and all day to get there.. LOL.
Anyway, knowing as hot as it gets in the dessert during the day, it gets butt-ass cold at night as well. Wanting to know who has done what to help insulate the back, because that's where I plan on sleeping.
I plan on getting a 3/8" thick Westin bed mat. Figure that's gotta help, plus helping to not kill my knees crawling in the back.I've also read about some fiberglass mats they say insulate well also.
For some heat (If needed) I'm running power (On it's own fused circuit, connected straight to the battery) line that will run under the the truck, attached to the frame, coming up through the drain hole in the front of the bed. That will be attached to a dual outlet, cigarette lighter style.
That allows me two things. 1.) To attach one of those 12v heaters, and 2.) to plug in a 180w inverter for running my laptop. (Already have a 1000w inverter mounted inside on the inside rear of the cab)..
The shell is, of course, fiberglass with some carpet-like stuff all on the inside. (Looks good actually... Snugtop did a good job on that)... Guessing there's not to much I can do to beef up that, but any tips would be appreciated.
Have two air mattresses, but those only last so long.... No matter how 'good/HD' they say they are. Want to get one of those nice thick pads, like the ones that they make for cots. Gotta be better than an air mattress. And I've got a decent sleeping bag, but it's only a summer bag. Plan on getting a 3-Season bag.
Anyway, anybody have some tips or better ideas for heating, or especially insulating?

 

Cabelas 0° sleeping bag. You’ll be plenty warm

 

Thanks... I'll check it out!