Agreed, the left has real ideas for solutions to middle America. But these ideas are not what people want, so the burden is to prove that they work. Unfortunately for many of those ideas, such as universal income, it is still too early to say. Meanwhile other ideas, such as the current implementation of healthcare, is actively being rejected by voters after it was tried, i.e. it does not work (they're like a product customers do not buy, even if it's good for them).
You're also right that Trump's voters all not all poor and living in trailer parks (they also weren't all old people). I haven't seen data on socioeconomic background, but I wouldn't be surprised if they were essentially middle of the road. But for good reason, they are afraid for their economic futures. It's a justified fear, and it's good cause for feeling desperate.