NEW YORK (AP) _ Fishing gear, fast food, car rentals, camping trailers. America's 1986 summer travel boom will help those industries and many more while injecting billions of dollars into the economy and creating thousands of jobs, economists say.

''It's a very unusual year because there are so many forces coming together,'' said James V. Cammisa Jr., who publishes Travel Industry Indicators. ''I've never seen this in 15 years in the business.''

Among those forces, say economists, are falling fuel prices, the dollar's lower value, declining interest rates and fear of terrorism abroad.

Cammisa estimated the domestic travel industry would enjoy an unexpected $15 billion boost this year, including perhaps $1 billion that would have been spent by Europe-bound American tourists who decided to stay in this country.

The bright outlook has been compounded by expectations that at least 22 million foreigners, the most since 1981, will tour the United States this summer, lured by overseas currencies up to 40 percent stronger against the dollar than a year ago.

By some estimates, the travel and leisure sector of the economy comprises about 9 percent of the gross national product. That put it in excess of $287 billion last year.

Definitions of the travel and leisure sector vary. Some economists say it includes hotels, restaurants, transportation and sporting goods; others say it extends to luggage, gardening tools, clothes, cosmetics and such luxuries as compact disc players and hot tubs.

''There are billions of dollars at stake, from the automotive industry to blue jeans, from swimming pools to pup tents. It's hard to find the end of the residual falloffs,'' said David S. Leibowitz of American Securities, a New York investment firm that follows the travel and leisure industry.

Kurt Brown of the forecasting firm Data Resources Inc. in Lexington, Mass., said airlines, trains, buses, hotels and restaurants likely would benefit most. He estimated the value of purchased meals and beverages alone would jump from $142 billion in 1985 to $150 billion this year, while intercity travel will increase from $86.5 billion in 1985 to $96 billion.

''There will be a lot of jobs for students,'' he said, ''and to the extent that you get dollars into the economy, the net impact tends to multiply itself through. For instance, a student gets a summer job at a fast-food restaurant and buys a car.''

The recreational vehicle industry is expected to benefit from the domestic travel surge. ''With all the economic indicators aligned so perfectly, we can't see where we'll go wrong,'' said Janice Conway, spokeswoman for the Recreation Vehicle Industry Association, based in Reston, Va.

January shipments rose 11.4 percent over the same period in 1985, she said.

The Brunswick Corp. said it expected a good year for its Mercury outboard motors unit, which accounted for two-thirds of its $1.5 billion in sales in 1985.

The Zebco fishing gear subsidiary of the Skokie, Ill.-based company already is heading toward a record year, said Brunswick spokesman Ross Stemer. ''People are saying, 'Wow, maybe I'll just stay home and fish this summer rather than go to Europe and get shot at,''' he said.

Still, some economists warn against overestimating the benefits of a travel boom - noting that the overall economy has languished, marked by weak gains in income, employment and consumer spending and that some states are suffering because of the declining petroleum industry and farm-belt crisis.

''Consumer optimism has been kind of wavering,'' said Sandra Shaber, vice president of consumer economics at Chase Econometrics of Bala Cynwyd, Pa. ''So while people may think this is a good time to travel, some might decide it's the better part of valor to stay home.''

Saying that, she still estimates the travel and leisure sector's 1986 expansion will exceed 5 percent, much faster than the overall economic growth rate.