If the spoke breakage is from fatigue then yes, there is a threshold of wear before they break. Sapim is a respected manufacturer, but they make a variety of products at a variety of prices. If you look at their website, they give the results of their own fatigue testing for each spoke they sell. The cheaper ones definitely don't last as long as the more expensive double butted stainless steel ones.

If your spokes were too tight (not likely), the wheel would be at risk of collapsing into a potato chip or taco shape. If the spokes are too loose, unevenly tensioned or both the rim would flex more as you rode, and the spoke elbows would fatigue and break.

Are your spokes butted (thinner in the middle, thicker at each end) or straight gauge? Sometimes you have to pinch the spoke between thumb and forefinger and run your thumb and forefinger up and down the spoke to find the difference. Butted spokes tend to absorb impact better than straight gauge, so they last a lot longer. They also cost more to make, so they tend to be found on much more expensive bicycles, or on expensive handbuilt wheels.

If the replacement spokes are holding up, keep replacing spokes. When you get some place with a good bike shop, consider having the wheels rebuilt with better spokes. For your next tour, start with properly built wheels with double butted stainless steel spokes (Sapim and DT are both good). Also consider getting a rear wheel with offset spoke holes. These will spread the load more evenly among all the spokes. Wider tires will also help cushion the spokes.