Minimum wage work in the US is brutal. The federal minimum wage is $7.25, amounting to $14,500 a year if you can manage to work a steady job. That leaves you very little money left over for traveling to those national parks.
Being a janitor is actually a step up from a lot of retail jobs, where your schedule is variable and it's hard to work the same 40 hours per week. It might even come with health care, another thing rare among minimum wage jobs. It's less likely to come with retirement savings, and at $14.5k per year you're not putting anything away. Certainly not if you also like to take vacations.
Being poor in the US sucks. Being janitor is not the worst job, but from context I think they meant it that way. I think few people would take janitor in the US over engineer in the UK, and practically nobody sane would take jobs in retail or food service if they had any other option. (Part of the reason they pay so badly is that those are the jobs occupied by people who have no other option.)