Where are all the seagull nests?
Breeding season

Chances to see nests will be higher during breeding season which normally occurs once a year. Eggs will be laid in early May but they will be built in the previous months.

Area

In densely populated urban areas gulls may build their nests on rooftops instead of ground nesting. In the case of New Jersey I found the following brochure: 

https://www.nj.gov/dep/fgw/ensp/pdf/bnb_brochure.pdf

If you mean seagulls as in all Laridae, then you have two members of that family (black skimmer and least tern) in that brochure that actually use NJ's beaches and even have their own designated space for that.

Googling 'seagull rooftop nests' brings a decent amount of pictures of rooftop nests. 'Seagull nests' brings a lot of cliff-placed nests, rooftop and ground nests. I assume the choice depends mostly on food availability and predator population density. I remember in Spain a few years ago there was a big enough problem to appear on local TV news related to seagulls attacking people. They would nest on every roof and attack people that would go out on the balcony with a sandwich.



Sources:


Habitat Selection and Nesting Biology of Roof-Nesting Glaucous-Winged Gulls
Kees Vermeer, Damian Power and G. E. John Smith
Colonial Waterbirds
Vol. 11, No. 2 (1988), pp. 189-201 https://www.jstor.org/stable/1521000?seq=1