1997 Chevy Blazer squalling belt

Had a squeal in fall last year after having a tuneup done in spring. Replaced the alternator and didn't help, replaced the pulley and didn't help. Finally replaced the belt and it squalled worse--got new belt and was ok. Then took 1800 mile trip end of June and now have the squalling again. Belt dressing helps but only temporary, next morning it is back. Can drive during the day and noise gets less but in morning is back to worse.
Can anything be done sort of changing the belt again? The repair book --Chilton--says is tensioner that does not need tightened but is that problem? This has a Vortec but every thing is working good, just squeals when running ---and especially after sits for 8 hours or more.
Thank you for your help.