1999 Chevy Tahoe 99 Tahoe engine not getting up to temperat

I've been having this problem for about a year where the engine temperature, according to the dashboard gauge, does not get up to the halfway point. It gets, at most, one quarter of the way up the scale. Also, I frequently (with each gas up) have to add antifreeze to the system though there are no leaks or smells of burning antifreeze. The car seems to run fine so I haven't been too worried about it, but the heater does not get very hot and I figure its connected and winter is coming. I replaced the thermostat and flushed and replaced the coolant and no improvement. I had the local mechanic check it out and he said at first look nothing looked out of order but he could run advanced diagnostics for $100 to see what is wrong. It seems to me like a sensor or gauge issue, in which case I'm not worried, but if is anything that may leave me stranded I'd like to get it fixed. Can you think of anything else that may be going on here?

Thanks.