To answer the question, how seagulls found their way inland in America, is to first know why they fly inland in the first place. Seagulls fly inland mainly in search of food. The more readily available the food is, the more they will fly inland to get it..
For instance, in the great insect invasion in Salt Lake City, Utah in the late 1800's, seagulls flew in from California and gorged themselves on these insects and so it was claimed a miracle from God by the Mormon religion.
Seagulls must have made a great impression in Utah because the seagull is now known as the Utah State Bird. They are considered a scavenger bird and will follow a farmer's plow as it turns the earth over for planting. The seagulls swoop down in great groups to eat the worms and insects that have been uprooted.
As America became more urbanized, this provided a greater reason for seagulls to fly inland. The simple principle is where there are people there is food.
LANDFILLS: Seagulls are particularly attracted to landfills. The reason is obvious, they are searching for food. They have very sharp beaks which can easily break open plastic trash bags. They spill out the contents and eat what they can find. This includes small rodents who are also known to occupy landfills.
GARBAGE CANS: It has been said that seagulls are so smart, they know when to raid your refuse when you put it out for collection. If it is put out the night before, as most folks would think to do, they make their entrance just before dawn, when they know the garbage trucks are on their way.
FAST FOODS: Seagulls like fast foods, too. They fly inland to watch for a dropped french fry or a spilled food item. They like to scavenger hunt during the day and night. Or, they can easily open a fast food bag dropped or thrown on the ground and check out the contents. They are attracted to fast-food establishments because they can smell it a mile away, like us humans!
HABIT: Birds of a feather, really do flock together. When a few seagulls spot a good place to find free food, their companions, always searching as well, see a friend fly down quickly in hopes of grabbing something to eat. This activity will spur others on to quickly fly down to grab something too. However, when it comes to sharing, they are not exactly friends.
The conclusion is clear. Seagulls fly inland to search for food. They are not fussy when it comes to selecting their eats. Be it landfills, restaurants or your uncovered garbage can, they will quite happily gorge on whatever is available. While seagulls mostly dine on fish, or hang around fishing docks looking for scraps of fish, they are not afraid to catch a ride on the air currents and dine inland...where the pickings may be easier.