The relatively short lifespan that you refer to is for the most part a rather unfounded lie, spread by dissidents of the Empire and outside the forces. The lifespan on a given world, for the most part is quite extensive, with most slaves lifing to a comparatively old age (most to their late 40's at least). There are some exceptions, where the planetary enviroment is extremely hazardous to all life, but again in regards to the taks they are given more then adequate saftey equipment is often issued. Amarr groups such as the SCPS have shown a willingness to undertake to monitor and improve the lives of slaves under the jurisdiction of the Empire, keeping them in a sufficiently habitable environment. If you would care to compare this with a relative lifestyle with some of the more liberal nations, I would be interested to find a comparable mortality rate and standard of living amongst commoners.
The slave stocks for the majoity of Holding s remains quite strong, this is proven by the lack of dependance on slave farms as each colony is self supporting in that matter. The need for adequate living conditions as part of the Minmatar stock innate breeding ritual, shows quite well as an adequate indicator to a given slaves surroundings. For example, a colony with a low population is more often then not in need of an urgent review of what it can offer to its native group. If I am correct in my thinking, news of depleted slave stocks in a given colony are sent to the SCPS for immediate inspection? This is something that you will obviously need to refer with Jim Hawkins as the orginisations founder. This by its very nature, ensures that the teaching sof one generations can be imparted to the next, through suficent numbers of educated slaves.
The above therefore sets a precident for a relatively good stanard of living for Imperial slaves, I hope this answers that aspect of your question.