when to replace fan belts

Let's see - I have a 1999 (bought new in July of 1999 - National RV Tropical with tag axle) chassis v10 with a 129000+ miles & 14 + years old. I am still running all the original hoses and the serpentine belt. I carry spares and I check them regularly (as recently as 4 months ago) and unbelievably they all still look and feel excellent. I am sure they will fail eventually but we do not travel nearly as much as we used to (maybe 7000 - 8000 miles a year) and unless I see that they do not look right when inspecting I am leaving them alone. I'm thinking they may last until I quit RVing, ha. By the way I did have the plugs changed around 94000 miles just because that was not something I wanted to have to do on an unplanned basis, but the engine was running just fine at the time. I believe that this particular RV by National had the cooling air flow around the engine, either accidentally or by design, dialed in about as good as it can get. Other than the plugs and routine lubrication this engine has required zero maintenance.