When towing 1000 lbs I drop 8mpg. is thois normal?

Seems a little excessive. Are you going up hills in overdrive? That's not a heavy trailer. But are you pulling it uphill in Overdrive? The strain on the transmission from not enough rpm's is worse than letting the engine run faster, the trans is not working as efficiently. Cruising along level. O/D is fine over 60 mph or so. But slow or uphill taxes both unless you drop a gear.

And are you sure about that weight? A 14' long trailer has gotta weigh more than 200lbs.....doesn't it?

I am going up and down small hills but nothing that puts the trans in overdrive. I just checked the weight of the trailer and I'm guessing that it is the GVWR (2900lbs) minus the full load rate of 2010lbs so that would make the trailer weigh 890lbs (I guess) with the 600lb load so a total towing weight of 1490lbs. the MPG drop still seems a bit excess.

Also, this drop in MPG is at highway speeds 60 to 75 mph.

No puedo colocar un camión UD diessel 1998 buen precio

Tow capacity specs, this shouldn't be a challenge for your situation, https://www.edmunds.com/chevrolet/silverado-1500/2015/?tab- id=specs-tab&sub=

This drop in mileage is probably not out of line depending how you drive and how the air pressure is maintained in your tires and the speed you are travelling. Hand calculate the fuel mileage for a full tank and do not rely on the bozoputer in the truck for exact fuel mileage.