The data I have seen points to new US military recruits, at least in recent years, being predominantly "middle class" basically to say they aren't people living in poverty, but are not coming from notable generational wealth either.

It is hard to get exact data, but I have seen multiple sources suggest that the $50k - 100k household income bracket is the biggest for military enlistment. Like this study for example

The people that I knew growing up who have since gone into the military (and I no longer have contact with) did it because someone in their family was in the military, none of them were poor. I think the whole propaganda and perception of the military being a force for good and virtuously defending the country plays a much bigger role than the money aspect, even though that perception, as we know, is extremely inaccurate.