Straightforward USA map with lat/lon grid.

usa_grid_map.R

library(tidyverse)

basemap = map_data('state')

lat_range = c(20,60)
lon_range = c(-150,50)

ggplot() + 
  geom_polygon(data = basemap, aes(x=long, y = lat, group = group), fill=NA, color='black', size=1.5) +
  scale_x_continuous(breaks=seq(lon_range[1],lon_range[2],10), minor_breaks = seq(lon_range[1],lon_range[2],1)) +
  scale_y_continuous(breaks=seq(lat_range[1],lat_range[2],10), minor_breaks = seq(lat_range[1],lat_range[2],1)) + 
  theme_bw() +
  coord_fixed(1.3) +  
  theme(panel.background = element_rect(fill='white'),
        panel.grid.major = element_line(color='#0072B2', size=1),
        panel.grid.minor = element_line(color='#56B4E9', size=0.3),
        axis.text = element_text(size=25),
        axis.title = element_text(size=30)) +
  labs(x='Longitude',y='Latitude')