Best transmission/model for towing

I have been using my 1980 300sd to pull a small enclosed trailer (w/ nosecone, 1000 - 1500 lbs). I have had no problems with the transmission whatsoever doing this for the past 80 thousand (163,000-241,000) miles running on veg, but this vehicle was just totaled by an suv so I am looking for a replacement.
I would like to have a wagon for the extra space but I'm worried about putting too much strain on the transmission. I'm also wondering if the SLS will be a good or bad thing. I was hoping it would be good as the 300sd tended to sag a little with 100lbs on the hitch.
I changed the transmission fluid/filter every 15,000-20,000 and used valvoline maxlife, but had always considered adding a temp gauge and an additional transmission cooler. I really love the Benz pulling trailer running on veg setup but I do not want to get into trouble with a new vehicle. Thanks in advance for the help on this.

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