Sounds like Intake manifold problem on Venture 2002? Affects millage too?

We were planing to sell our Venture (65,000miles) this year, so I decided to inspect under the hood. I noticed there was no coolant in the reservoir. So I opened
the radiator cap, and notice a sludge in my coolant. Looking at my oil, there is no sign of coolant at all. I think the reservoir has been empty for a long time now, maybe half a year or one year. But I have never had it overheat, strange enough and never seen any leaks on the driveway. Those this sound like it's just a slow leak? Is this something that warrants being fix before selling? Or is it a major issue that could have cause some serious damage to the engine?
Also, I have been getting very poor gas millage. Can a head gasket affect millage in any way? Thanks.
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