Yeah i agree, something is wrong with your truck, or the trailer.
I tow a 6' wide x 8' long truck bed trailer with wood racks with a cord and a half of Firewood, 4 times a week, with a 1/2 cord in the back of the Truck.
I only have 3.55 gears, some heavy *** 33" nittos and i dont have a problem pulling that at 60, and i still have the power to pass semi's without problems. and i get about 10-11 mpg

I have towed my buddies car hauler, with his Rock crawler, prolly wieghs around 8K total.
and it pulled it fine, you could feel it on hills, but it would still pull it up 6% grade at 55 in 2nd gear. in 2nd gear at 55 i think i run around 4500 rpm. averaged about 10 mpg

and Ive also towed a 6500lb equipment trailer, with a 78 full size lifted chevy, it was heavy got about 8 mpg, but it pulled it fine, you knew it was back there.
So just get your truck checked out, if its fine better check the trailer

Oh and VP64, Ca has the 55mph trailer limit, some other states dont, like nevada its 65 with or without a trailer.

 

I just arrived in North Dakota from southern California (1900 miles) pulling a 16' cargo trailer that got an official weigh in of 7,954 lbs. this truck is far from stock (look at the sig) and I had absolutely zero problem with power. Crusing on level or even slight up hills I stayed in overdrive at 2100 rpms at 70mph. The steepest climbs going through the mountains of utah (7000+ feet) I never even left 4th gear (65 mph 3500 rpms). Only once did it drop to 3rd gear up a climb and that was a 2,000 ft climb over 8 miles. I stayed at 55mph and rpms were around 4k if i recall.

 

my 05 Ram QC pulls my 18ft tandem axle trailer both empty and full all the time. I have had it loaded with everything from frontloaders to wood to ATV's, etc... Most times I have to remember I have a trailer behind me. At least with my old ferd I couldn't help but remember I was towing something. rarely ever go above 3,000 rpms.