I too did not want to sell a perfectly-running '99 Nissan Frontier 4-cyl 5-spd manual when we got interested in camping. We bought the 13'
Scamp Standard but loaded. It always weighs apx 1800 lbs on trips.
If I was pulling in flat land, I wouldnt hesitate to pull a 16'
Scamp. But, I live in the "Hills of Tennessee" and I wouldnt even THINK of pulling anything larger/heavier with my
Scamp. Yes, my truck is rated at 3500lbs also and my trailer weighs HALF of that nearly but I STILL would not pull a larger trailer with it. Believe me, on some of the hills I hit here, you WILL rev that little motor. That doesnt bother me until I hit some of the larger hills as well where they can be 2-3 miles worth of long climbing.
As far as automatic vs manual, I wouldnt pull with the combination I have with an automatic transmission. Someone mentioned you wont be "burning clutches"? I'd MUCH rather burn out a clutch than to strip/burn out an "automatic" and have it repaired! I also did not have to add a "cooler" with my manual shift- that tells you something there.
But, as always, this is my "experienced" take/opinion on pulling with a 4-cyl.
One final word of advice, what EVER you do, do **NOT** "lug" that 4 cyl motor!!! Keep the RPM's up and you will be fine.