What this rule is meant to represent is that living and working on a small cramped poorly designed ship (or more precisely one with a high crew to crew space ratio) will eventually have a negative impact on the mental health of the crew.
This certainly doesn't mean full on flip a switch insanity. It can start out as poor morale and wear away at the crew until somebody snaps, either going postal or quitting or getting fired at the next port of call. Most likely an extreme case where a character "becomes insane" i.e. San= 0, they receive a Mod on tasks based on Edu or Int and the difficulty is Uncertain.
Only if Tension becomes negative will the crew have to make a San Check and then only if a crew member's San is abnormally low (less than 5) are they anywhere likely to fail. Each failed San Check only reduces Sanity by one level so to go "insane" a crew member would have to fail 4 San Checks in a row.
The whole situation can be avoided by providing a spacious common areas and staterooms, by prescribing medication, or by giving the crew access to counselling via a Counselor or a special console that provides counselling services.
[EDIT because I confused Ship Ergonomics rating with Ship Comfort Rating]
A World War 2 Liberty Ship had a gross register tonnage of 7,191 tons which equates to approximately 1440 Traveller dTons. The ships had a peacetime complement of 42 (for detailed breakdown see Liberty Crew at this site:
http://www.usmm.org/libertyships.html), and carried between 12 to 27 additional Naval Armed Guards during wartime, for a total complement of between 54 and 69. They had a cruising speed of 11 knots. Assuming a voyage in peacetime from New York to Southampton in the UK, they covered a distance of 3200 nautical miles in about 12 to 14 days, depending on the weather in the North Atlantic Ocean. In winter, that is viewed as the most difficult ocean to sail across on the planet. If sailing from San Francisco to Brisbane, Australia, the travel time in peacetime would about double to 24 to 26 days, while wartime voyages would last a month or more. The ships were not designed for comfort, but for the maximum cargo capacity at the lowest cost.
With the full wartime complement of 69, the ship's were crewed at the rate of 1 person per 21 Traveller dTons, with the peacetime complement, the crew rate was 1 person per 34 Traveller dTons. In rough terms then, 5 persons per 100 dTons in wartime, and 3 persons per 100 dTons in peacetime.
What manning rate are you assuming in your Traveller 5 universe which is going to exceed those manning rates per Traveller dTon, considering that the quarters on the Liberty ship were far less spacious and comfortable than you would get if you designed a ship under Traveller rules. Also, the voyages lasted anywhere from twice to 4 times or more as long.
Then you have the accommodations for sailing ship crews, for which information can be found from reading the opening chapters of Frank Bullen's outstanding book,
Cruise of the Cachalot, which can be downloaded from Project Gutenberg. You also have the crew lists in Theodore Roosevelt's,
The Naval War of 1812, for the
USS Constitution and a ship-rigged sloop of war. The 44-gun frigates were rated with a crew of 400 men, the
Constitution had on board 456 in its combat with the
HMS Guerrière. I will not even get into the dense manning onboard of US Fleet submarines in the Pacific during World War 2, nor of the overmanning of some of the destroyers in World War 2.
Are you claiming that the conditions on a standard ship, built under standard Traveller ship-building rules, are so much worse than the examples cited that a Jump of 1 week may cause sanity problems? If you are, then kindly spell out quite clearly those conditions.
I repeat, my "willing suspension of disbelief" totally vanishes at this point.
. 
