Sounds like a common problem with our vehicles. When I was up in ME, the temp was at 0 F and the thing never got warm. Even now, when I'm on the highway, the heat is barely there. I threw in some Prestone 50/50 antifreeze and it helped a little bit. I have read somewhere that you should usually go 50/50 in the summer and 70/30 in the winter and I do think that is a good idea that I might try in the future. But it may be the t-stat. idk
Another thing to try, and this is gonna sound stupid, put a good size piece of cardboard or plastic or whatever infront of the radiator to block the cold air. My dad suggested it to me, and I thought it was a stupid idea at the time. My dad was a truck driver and he said that's what they did in the winter time to keep the temps up there. I guess it works very well on the highway and they just throw fabric of some sort on the front grill. Next time you see a truck look for it. Most will have it. It must do something because these trucks last 500k miles easy. just remember to take it off when the temps warm up lol.