Ok, in summary, Mark Twain is not a WP:RS because there are accurate estimates of concomitant population, which is contradictory to your earlier statement that There are no accurate estimates of population at the time so laregely scholars can relate to the observations of travelers at the time.  This reference and others like it breech WP:RS by not conforming to the subsection Beware false authority

Authority of authors   
Prefer authors with an established reputation in the field where Wikipedia uses them for reference. 

Mark Twain is a satirist and a writer of fiction, not a respected demographer.

These references are also in breech of 

Exceptional claims require exceptional evidence

Claims not supported or claims that are contradicted by the prevailing view in the relevant academic community. Be particularly careful when proponents say there is a conspiracy to silence them. 

There is no conspiracy to silence these claims.  This account is problematic because Twain's work seems to indicate that the 350,000 people do not exist in Twain's time, or that there are much less people than 350,000 living in Palestine.  Verified and peer-reviewed sources indicate that there were 350,000 people in the area concomitant to Twain's visit.  Thats what needs to be said in a section on demographics, not the musings of a satirist and fiction author.

Self-published sources

Main article: Wikipedia:VerifiabilityA self-published source is a published source that has not been subject to any form of independent fact-checking, or where no one stands between the writer and the act of publication. It includes personal websites, and books published by vanity presses. Anyone can create a website or pay to have a book published, and then claim to be an expert in a certain field. For that reason, self-published books, personal websites, and blogs are largely not acceptable as sources.Exceptions to this may be when a well-known, professional researcher writing within their field of expertise Twain's work is also in breech of this portion of WP:RS as well.  His accounts cannot be considered verified or subject to any form of independent fact-checking.  He is also not a well-known, professional researcher writing within their field of expertise.

If we were doing original research or if we did not have good population estimates, then this information would be valuable. Since There are no accurate estimates of population at the time so laregely scholars can relate to the observations of travelers at the time'' is clearly wrong and we do have accurate estimates of population at the time, it is clear that we need to use these and expunge the article of all non-scholarly sources, including the Herbert Samuel link I contributed earler.