Low roader can be formed in several contexts by taking the generic phrase low road, that is, someone who takes the difficult course or the ethically troubled course (cf. Merriam-Webster, high road) and applying the generative suffix -er, a person who is associated with phrase (e.g. old-timer, early-riser, bucket-kicker;see M-W)).
For instance, here is the phrase used in two different recent contexts. First, unsavory debate practices:
The low-roader can dump so much mud and blow so much smoke that the high-roader has no chance. What's the answer? Should you forget logic and evidence and fight just as dirty as your opponent? No, what you do is to refuse to play the low-roader's game. (Parsons, Keith. Rational Episodes: Logic for the intermittently Reasonable. 2018)
Second, where low road writer is used to describe someone who does not reinvest gains in writing efficiency back into writing, low roader is also used:
The low road writer is likely to spend freed-up channel capacity fine-tuning the [writing] operation (adding small details, changing occasional sentences and the like). Alternatively the low roader's attention might be spent on some activity entirely unconnected with writing, so that the effect of automisation is to decrease the amount of effort devoted to the writing task. (Expertise in Second Language Learning and Teaching, 2005)
The usage isn't standardized or in common dictionaries. Instead, it depends very much on what the user considers to be low in context.
While I can't track down a 1930s quote that corresponds directly to what you saw, the expressions high road and low road would've been out there, so it's possible you encountered someone adapting low road. See for instance this poem in the Kusko Times (1 December 1934):
The high road, the low road,
The path of joy or pain
Given the generic nature of low road, it is unlikely that low roader was a set term used to refer to "unsavory Americans abroad." More likely, that was a specific context in which low roader was formed as a nonce term.
