WTF ? That trans guy is not in touch with reality. I blow the trans on my 1991 C2500 and ended up rebuilding it myself because I learned enough to figure out most all the trans shops around me where idiots. Temperature is one of the top reasons trans fails. When towing or hauling up a steep grade the first thing you should do is watch the gauge and there are charts showing what happens at what temp. I have pulled over and let the trans cool down to avoid damage. But without a gauge your running blind and have no idea whats going on. Hell I could go on and on but get a gauge and send your trans man to this post and I will help educate that fool !!!