Freightliner Corp. stole the show in the heavy truck market last month, posting a whopping 30.5 percent sales increase as the industry eked out a slim gain.

The strong growth at Freightliner marked the industry's only solid advance last month as most of the major manufacturers lost ground. Analysts believe the three-year boom in heavy trucks has finally peaked, although order backlogs are still keeping factories at full bore.The September sales jump from a year earlier came on Freightliner deliveries of 4,806 trucks for the month and 39,665 vehicles so far this year. Sales in 1995 are up 20 percent at the industry leader from Portland, Ore. It has 26 percent of the market, according to U.S. retail figures from the American Automobile Manufacturers Association.

Industrywide, September sales of 16,434 units rose 3.7 percent to a new high for the month, while record year-to-date deliveries of 152,604 vehicles climbed 11.9 percent in the heavy, or Class 8, category, defined by gross- vehicle-weight ratings of more than 33,000 pounds.

Paccar Inc. of Bellevue, Wash. continued in second place, based on sales of its two divisions, Kenworth Truck Co. of Kirkland, Wash. and Peterbilt Motors Co. of Denton, Texas.

At Kenworth, September sales of 1,949 trucks were down 0.5 percent from a year before. Sales of 17,873 units so far this year remained 7.1 percent above year-earlier marks, yielding an 11.7 percent market share. Peterbilt sales of 1,522 vehicles last month dipped 11.7 percent from a year before, while year- to-date deliveries of 13,863 trucks rose 1.8 percent for a 9.1 percent share.

Navistar International Transportation Corp. of Chicago held an 18.4 percent share as September sales of 2,682 units declined 12.3 percent, while sales of 28,095 trucks so far this year gained 9 percent.

At Volvo GM Heavy Truck Corp. of Greensboro, N.C., sales of 2,021 vehicles edged down 0.3 percent last month. Year-to-date deliveries of 17,582 trucks were 5.1 percent ahead of the 1994 pace, giving the company an 11.5 percent share.

Mack Trucks Inc. of Allentown, Pa. also captured 11.5 percent as monthly sales of 1,601 units dropped 4.4 percent, although sales of 17,484 trucks through three quarters rose 19.3 percent.

Ford Motor Co. sales of 1,591 heavy trucks advanced 4.3 percent last month, boosting sales of 15,586 units so far this year, to a 10.4 percent gain, for a 10.2 percent share.

Industry sales were mixed in the lighter categories of commercial vehicles, associated with distribution and the service sector.

In Class 7, rated from 26,000 to 33,000 pounds, September sales of 8,018 trucks fell 4.1 percent as year-to-date deliveries of 82,925 units are up 14.8 percent from the comparable 1994 period.

In Class 6, rated 19,500 to 26,000 pounds, September sales dropped 10.6 percent, to 1,458 trucks,although nine-month sales of 18,492 units are ahead of last year's pace by 15.4 percent.

In Class 5, rated from 16,000 to 19,500 pounds, September sales of 343 trucks slid 17.1 percent, dragging down sales of 3,122 units so far this year by 5.8 percent.

Monthly sales of 3,818 vehicles were up 6.8 percent in Class 4, with ratings from 14,000 to 16,000 pounds, but sales of 39,199 units lagged the year-earlier period by 5.2 percent.

In Class 3, rated from 10,000 to 14,000 pounds, September sales of 2,607 vehicles slipped 6.5 percent as year-to-date deliveries of 29,304 trucks fell 12.2 percent.