Keeping warm using air mattress

Have you tried a foam topper?

We have used electric mattresses pads for over 20 years and never had an issue, both S&B and RV.

Second the heated mattress pad. Much better than an electric blanket.

I would get a memory foam mattress pad to put on it. When we tent camped and slept on an air mattress, I was always cold for a couple of hours until the air inside the mattress finally got warm from our body heat. We ended up with a sheep skin pad on it, but currently use a memory foam mattress pad on our bed ( from Walmart - no cool gel) and it keeps you cozy.

Use memory foam on the air mattress or just the memory foam by itself.

Open up a sleeping bag and use it as a mattress pad. Rolls up easy the next morning.

My first question is what temp are you keeping the rig at night? Do you heat it solely with the furnace? If you say you are reluctant to use an electric mattress pad, or blanket is it because of the "cautionary" tales on here only? Im going to suggest that there are a LOT of rv.net users that rely on ceramic heaters, electric mattress pads and blankets on an ongoing basis. I would get her a quality electric mattress pad with a shut-off and give it a try. How about the hot pads filled with wheat that you heat up in a microwave and then place next to you as you sleep. Hot water bottle maybe! Lots of ways to keep folks comfortable.

Part of the issue may be the windows above the couch, if your couch is located like mine. If you will not consider a heating pad then try diverting a heating vents discharge air to under the couch.

We use an electric blanket on low under the sheets and mattress cover. Works great and runs well off of the battery.

If you are boondocking the above solutions may not work. I have a zero degree mummy sleeping bag in my rig. I use it when boondocking. It's gotten down to the 20 degree mark and I still stay warm. Also, throw some iron oxide hand warmers into a sleeping bag and you raise the temperature in the bag by about ten degrees.