Note: I have never used a RTT, but I routinely tent camp and backpack well below zero.
Focus on personal insulation as much as possible.
1st, and probably most important, is the mattress or pad you use. Modern mattresses are rated with an R-value. Look for an air mattress or pad with an ASTM F3340 compliant R-value above 6.
Here is a really good multipurpose camping pad:
https://www.rei.com/product/148193/rei-co-op-camp-bed-self-inflating-sleeping-padYou probably wouldn't want to take it backpacking but for most kinds of all season car camping it will work great.
The pad is very important because much of your heat is lost through conduction through the ground or whatever is underneath you. When you lie in your sleeping bag, you compress all of the insulation underneath you, rendering it nearly useless. However, you only partially compress a pad, so it will retain most of its insulating properties. The pad becomes even more important when there is airflow underneath, such as on a cot, in a RTT, or in a truck bed.
Note that typical air mattresses are very poor insulators. Self-inflating pads are usually much better insulated.
Next is the sleeping bag. Modern sleeping bags are rated using the EN/ISO standard. The EN/ISO standard generally has a "Comfort" and a "Lower Limit" rating. The "Comfort" rating is what most people should buy to. The "Lower Limit" isn't a guarantee you'll be warm, just more of a recommendation that you probably won't die of hypothermia. (Chances are you will be uncomfortable at those temps.)
The ratings assume you use the sleeping bag in its most warm position, i.e., supine and with the drawstring pulled as far shut as you can while still being able to breathe. You can always open up the drawstring and unzip the bag to let some air out.
You'll want a bag that has a comfort rating no higher than the coldest possible night you expect. It's easier to open it up than to try and layer past its rating.
Here is a pretty popular bag that is EN/ISO rated:
https://www.rei.com/product/136215/rei-co-op-down-time-0-down-sleeping-bagNote that the Comfort and Lower Limit ratings differ from the advertised rating. This is typical across all brands.
If the bag is not listed as EN/ISO rated, chances are it uses a proprietary rating system that is not comparable to other manufacturers. These ratings are generally "survival" ratings, or ratings that say you probably won't wake up dead. If it is not rated, add about 15-20 degrees or so above the listed rating and pretend that is the comfort rating.
Here is a good inexpensive winter camping bag, but note it is not EN/ISO rated:
https://www.amazon.com/dp/B077NR4VCZ/?tag=wranglerorg-20closer.
This one would be great for winter camping near a car, but not so much for backpacking. I'm actually buying one of these for myself.
There are three major types of bag fills:
Synthetic - cheap, warm, bulky, and heavy. Retains much of its insulating value when wet. Tends to be pretty durable. Very popular among campers who don't have to carry stuff very far.
Down: Expensive, warmer, lighter, and more compressible. Great for backpacking. Loses much of its insulating value when wet. Generally doesn't last as long as synthetic, but higher performance.
Water repellant down: Most expensive, lightest, warmer, more compressible. Doesn't lose much heat if it gets wet. Like normal down, doesn't last as long as synthetic, but very high performance. Highly popular among backpackers and mountaineers.
The pad and the bag are by far the most important items. But there are other items that can help a lot.
Sleeping bag liners like the Thermolite Reactor and Reactor Extreme can provide a small boost in warmth. I would suggest they add roughly 2/3rds what they advertise as a rule of thumb.
A pillow with fill will generally insulate better than a fully inflatable pillow.
Wear a thick stocking cap to help add some insulation to your head.
Avoid wearing tight clothes like tight socks and tightly fitting baselayer that can restrict circulation, unless you like cold feet.
Bring a set of loose clothes solely for sleeping. Even in cold weather, you tend to work up sweat when moving around, and mildly damp underlayers will significantly impact your temperature.
Eat something sugary or carb-y before going to bed. S'mores are perfect, but even just a handful of random snack foods is good.
Never pull your face into the sleeping bag. Sure it feels great at first, but all of the moisture in your breath will condense and make you much colder.
Always hydrate before sleeping. It is far better to have to get up and pee in the middle of the night than to be cold because you're dehydrated. When you're dehydrated, your blood volume drops, and less blood will get to your extremities. (And if you wake up needing to pee - just GO. Trust me.) Bring an old Gatorade bottle if you know you'll need to pee.
