1992 Chevy Suburban Transmission slipping

This is a long story. Last September on a trip to Myrtle Beach the tranny on my 1992 motorhome went out. (I listed as a Suburban because there was no choice for a P-32 Chassis) I had it towed to a GM dealer n Red Springs NC. They replaced the tranny with a GM certified and warranted for 2 years tranny. When I picked it up and headed home, about 75 miles from the dealer while on a grade it backfired rapidly (I thought). There was a sound like a series of high speed backfires in the catalytic convertor area and you could feel some hesitation. I stopped and called the dealer and they said that they had not done anything that would cause this to occur. That I should go on home and let my local mechanic check it out. I have been trying to correct this "backfire" problem since then. It only occured intermittantly and at higher highway speeds when the engine was shifting. I spent over $1000 dollars trying to fix the problem. Last week we took the RV on a 400 mile round trip to and during the trip the tranny started slipping after any time that it had downshifted to a lower gear and then back into high gear, it would slip really badly in high gear. If I pulled over and turned off the engine for just a minute or two and then started out again it would be OK till another downshift occured. I got out my Chilton manual and read it and it said that slippage and possible transmission damage could occur if the tranny was overfilled. It said the fluid becomes aerated and causes slipping. So I checked. After running till the engine and tranny were hot and checking, the fluid was slightly higher than it should be, about an 1/8 in above the fill line on the stick. I confered with my local mechanic and he did not think that small amount could be a problem. But he suggested that I check to see if the mechanic had pushed the dipstick tube all the way down in the new tranny when it was replaced. I did check, and he hadn't. It was up out of the tranny housing about a 1/4 inch. Bottom line is, I made it right and with a small tube siphoned out tranny fluid and then refilled to the proper level. It turns out there was 1 1/2 quarts too much fluid in the tranny. On a test drive there was no noise (backfire) or slippage. Now for my question. Do you think this has damaged the transmission? I have driven between 4 and 5 thousand miles this way. I now believe that what I thought was a backfire was actually some kind of noise coming from the torque convertor. As the new transmission is under warranty, what would you suggest I do?