Finding parking is the hard part. Jared Rosenthal circles the block on 116^th Street in East Harlem a few times before he gives up and slides his RV  to the curb in front of a “No Parking” sign. After that, it’s just a matter of sitting back and letting his paint job do the talking.

The side of the van, in big, white letters, reads: “Who’s your daddy?” Inside, for as little as $299, Rosenthal offers onsite paternity testing.

A laid-back 30-something who sports a graphic T-shirt and dog tags engraved with his two children’s names, Rosenthal spends most afternoons rumbling around New York in the van, which he considers a sort of mobile billboard for his drug and DNA testing company, Health Street Clinic.

When he founded the business two years ago, he tried to kick up attention by painting a giant cup of urine on the side of the RV. “We called it the Pee Mobile,” he said with a chuckle. He commissioned the “Who’s your daddy?” signage a couple of months ago after noticing his clinic in the Bronx was getting a lot of inquiries about paternity testing. The marketing move boosted sales and attracted the attention of a television agent looking for a new reality show.

As technology has improved and prices have come down, interest in DNA testing has increased, said David M. Bishai, a professor of population economics at Johns Hopkins University. DNA labs such as Rosenthal’s processed more than half a million paternity tests in 2010, compared to just over 100,000 two decades ago, according to the American Association of Blood Banks.

Rosenthal hadn’t even taken the keys out of the ignition on a recent afternoon when his first potential customer walked up. A woman with a smart bob, clutching a planner against her blazer, knocked timidly on the window.

“I really like the van,” she said. She wanted more information about DNA testing for immigrants who want to bring relatives to the United States. While Rosenthal can conduct paternity tests in the van with a doctor’s prescription (required by state law), most people who stop by prefer to have a consultation, then set up an appointment at Rosenthal’s clinic.

While he chatted with the woman, a steady stream of people stopped to gawk. It’s not funny, insisted Risa Levine, a 26-year-old teacher — or at least it shouldn’t be. But Levine  had to swallow a giggle when she spotted the parked RV.  She snapped a photo with her smartphone.

A man leaned out his car window to catch Rosenthal’s attention. “Hey man. You going to be around here tomorrow?”  Rosenthal suggested he copy the phone number from the side of the van.

“When I read it, I was like, ‘Wait. What?’” said Aixa Valentin, who stopped to take a cell phone picture on the way to Best Buy. “I had to double-take to see what it’s about.”

While a day roaming the city in the “Who’s Your Daddy” van isn’t like the chair-swinging antics that play out on “The Maury Show,” the job does involve its share of drama, Rosenthal acknowledged. People cry. People plead. People try to bribe him.

One man offered him $1,000 to lose the condom his wife had found in the back seat of his car, he said. She wanted laboratory proof that the condom wasn’t his.

“He set the test up, then he called back from another phone to see if I could dispose of said specimen,” Rosenthal said. “He said, ‘Look man, I got a Mercedes. I got a house. I don’t want to lose it all. I’ll give you whatever you need.’ But, of course, we don’t do that. Ever.”

Rosenthal often plays counselor as people confront heavy questions: Did she cheat? Do I have a sister? Who is my father? There’s something about sitting face to face in the RV, insulated from the cars zipping by outside, that inspires trust, he said.

While he explains the mechanics of a paternity test — get a prescription, swab the cheek, send the DNA to the lab — clients unload their worries.

“It’s a little more intimate than a clinic,” he said. “It’s not a big desk with a couple men or women in scrubs sitting behind it barking out, ‘Next!’ Here, they come in, we shut the door to the RV. It’s kind of like being in the car with a friend. You just start talking.”

Using DNA testing, Rosenthal has reunited sisters and helped a 44-year-old Harlem man find his long-lost 20-year-old daughter. But the tests don’t always end happily.

“The hard ones are when the dads come in with a child — quite often they’re changing diapers in the office — and they give you their name, then they give you the child’s name and it’s the same name with Jr.,” he said. “You’re just like, ‘Aw, man. I really hope this kid is his.’”

It’s tough to deliver bad news, he said. “I’m sure it’s much tougher to receive it.”