I went roughly that far on the belt on our 2003 and when I pulled it off I could see the metal belting material through the "V" grooves in the underneath side. I'm thinking that 90,000 miles is a bit too far, and would expect the belt to fail pretty much any time beyond that point.
As for the tensioner, I replaced in at 60,000 miles on our 1998 due to excess noise (and the replacement was still on the engine and fully operable when I traded the van in with 170,000 miles on the clock), however, the tensioner on our 2003 is still the factory original part and operating up to spec.
Well, the question is "do I NEED it"? The answer is no, but you don't NEED a running van either.
I think it's easier, though, to change these simple things out on your time, not on the van's. Belt is like $25, and takes 20 mins to change. The tensioner is like $35, and maybe a bit harder (30 mins -- but requires some contortion skills to get out). I'd opt for the idler pulley as well -- another $20 (I had one of these spit the bearing balls -- not fun day on a trip with the Caravan that day!). Idler is easy (one bolt).
With 90K on the clock, I'd opt for the belt and probably the idler; I'd do the tensionser if it were making noise.
I usually change the belt and tensioner at around 100K. Changed the 97 at like 92K, changed the 03 the other day at 104K. I just do them both, cheap insurance to me.