Towing Fluid Change Interval

Does anyone have any advice about the interval for changing the Tranny and Differential fluids in my '98 1500 5.7L Vortec. I bought the vehicle used
with about 50K on it and it now has about 72K. I have been changing oil using Valvoline Synthetic at approx 3K intervals.
At about 57K we purchased a travel trailer that is about 4500 lbs loaded.
At 59 or 62K (I don't remember exactly) I had all of the fluids flushed and replaced. I used synthetics for all of the fluids where available (Ouch! $$$)
When I was in for the 71K oil change, the Service Tech said that the Transmition fluid was "duller" in appearance than it should be and suggested changing it. I have towed the trailer about 3500 miles and was thinking that this seemed like a shorter interval than I though it would/should be especially using the synthetics.
Any suggestions on estimated mileage? What would be the best way to test? I don't mind paying to change the fluids frequently if that is required under these conditions but then I also would question whether the synthetic fluids are to any advantage?
TIA Stev
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wrote:

Drain a sample of the fluid, then top it off.
Then send it to: http://www.blackstone-labs.com /
Oil analysis is only way to know for sure what the condition of the oil is. And beware, many service techs get bonuses for upsales.
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