You should have 24+ on a day trip like that. This time of year, check your tire pressure as it will loose 2 psi for every 10F drop.

You obviously have a manifold leak - somewhere. I assume you have the 5 cyl engine with a low pressure turbo. There are a lot of vacuum lines! Check the obvious ones at the front of the manifold, then the ones around the boost controller (4) and the turbo (3). Grab each of the intercooler lines and try to twist them, if any move even a little bit then tighten the clamp. Same with the air attachment at the throttle body and air cleaner.

At 80K, how old are your Volvo spark plugs? Are they clean?

The dealer should have put your engine on a scanner and checked the fuel and exhaust sensors. But a lazy mechanic won't bother. If it ain't broke...

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Keeping it running is better than buying new