Heater output in Outfitter apex8.5?

That does not sound right. I camp all the time at those temps (Sking) and my camper would be toasty. By heater is a suburban seq 16000btu unit made for TC's mostly. The air coming into the unit is quite warm but not really hot.Sounds like partly clogged main burner or something. Can you see burner through a peep hole?
Either way I would get it checked.Good Luck

Your SF20 is a Suburban 20,000btu furnace
It should be able to warm up/maintain a small trailer

Only warm exhaust means burner is NOT functioning properly.

Dirty, clogged/blocked burner slots, low propane pressure etc resulting in a very low yellow flame vs a STRONG BLUE (HOT) Flame

Here is an Installation/Operating Manual
LINK

Check the burner, make sure you're getting a full, optimal flame. My furnace in my Apex 8 stayed a bit on the cool side until I dove into it and re-routed all of the outputs (and closed off two of them). Make sure all of the ducts are sealed properly and there are no leaks. Look at the furnace enclosure and make sure none of the removable duct ports are missing. Mine had two that fell out and I was heating behind the cabinets. Reinstalled the 3" plates that fell out and viola, more heat/air coming out of the ducted vents in the camper.

What is your outside temperature? Well below freezing, I suppose?
Has it done this before?
Does the stovetop work as it should?
I had an Apex 8 from 2011 to 2016. 75 and up was no problem with freezing temperatures outside.
The problem may not be the heater but the fuel source.
In very cold temperatures, your regulator may be frozen.
I once had an LPG bottle that had the wrong mixture of propane/butane. Butane does not gas out under 30F or so. The bottle did not produce enough gas for heating when it was cold. Hence my questions above. If in doubt, can you change to a known good LPG source and test again?

X2 on your regulator as mentioned by Joerg.

Thanx all for the great replies. I will dig into it this weekend.
Nothing ventured, nothing gained.

I almost always get wasp/hornet nests in my furnace outlet when not using it. That's one thing to check.

x12 on the propane regulator. Does your stove work ok? That's a clue to regulator issues.

There's a few of us on the forum that have a lot of experience with Outfitter TCs. And are likely willing to help you with issues like this.