Jesse, sounds like you might have a vacuum leak. check SOGI,
www.supras.com > SOGi for tips to find vacuum leaks, or search
www.supraforums.com for the same
300k shouldnt be right, if you're using the stock fuel system. hell even i can beat that mileage.
first check vacuum leaks, then check your fuel filter, check all fuel lines, maybe check fuel pump, but first make sure your fuel pressure regular is working properly. im not sure how to do so, but check the TSRM @
http://www.cygnusx1.net/supra/library/TSRM/index.html
checking your AFM is going to be harder, but make sure the core is still solid and nothings broken inside, or covered in oil,etc.
if you can narrow down the problem a bit more, we can help you more.
g'luck!