Help! Dying rubber tree
Hello, new to the forums, was searching for help for my rubber tree. It's about 4 ft tall, in a 24" round planter about 24" deep, maybe a little more. It was doing great all summer, moved it inside in October before it got cold outside (it was out on the porch all summer). I've had it for over three years now and never had a problem until just this week. I also had a smaller one, about a foot tall that I had purchased at the beginning of summer. That one just died here a couple days ago, and I thought it was a one off, but now it seems the big one's going the same way, and I'm going to cry. The problem starts (it seems) with the leaves getting droopy. They haven't turned yellow or reddish in color, they just look all limp. So I examined the plant, and there are black splotchy spots on the green part of the stem/trunk (down lower where it looks more bark-like there are no spots, only up toward the newer growth). The plants were doing fine until just this week I noticed when this problem started. All of a sudden new growth just stopped and turned dark, when even just a couple weeks ago new leaves were unfurling. I water the plant about a half gallon (the smaller one received much less of course), maybe a little less usually about every 2 weeks. The smaller plant that's already dead it looked like the stem rotted from the inside out, it was all black and squishy after the last leaf fell off, again, at the top, not closer to the base, at the base and a couple inches above, the stem looked fine and wasn't soft. The two plants were not next to each other, one was in the kitchen and the other in the living room. It doesn't look like the pictures of the southern blight I've seen, and there is no fungus visible at the base of the trunks. There are no problems visible on the leaves either, no spots, bumps, tears, etc. No evidence of insects, unless they're under the dirt where I can't see them.
I will try to get pictures up tomorrow when the light's better for taking pics.