Quite funny the perception of struggling.
Does the typical 1/2 ton pull 3500-4000 RPM's while daily driving? Not consistently. But the same truck pulling it's rated trailer weight will pull those kinds of RPM's to move a load. It is NOT struggling it's working.
OP I think you have a gearing problem not a truck problem. But it's your cash and you have to do what you feel is needed to be comfortable.
I would like to say that if you buy a HD Gasser you are still going to turn some RPM's while towing. I went from a 1/2 F150 with a 5.4 to a 6.8 V10 in a F350 and I pulled nearly indentacle RPM's while towing the same trailer. The F350 only had about 2000 lbs more trailer towing capacity. But it had about 2 tons of payload :)