I have 1999 Ford F350 7.3 diesel I use the truck to tow a 30 ft gooseneck living quarters horse trailer. At normal temp the engine runs rough at an idle. When driving down the road at normal speed it runs good but when you slow down to 30-35the engine starts to run rough. I get 11 mpg pulling the trailer. When going up a hill and you give it the throttle it blows black smoke for about a couple minutes then the smoke quits. It also uses about 2 1/2 quarts of oil in 300 mile Can someone please help

There might be an idea or two in this link that will help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-8.html#post1415281

With that amount of oil consumption, I would have a good diesel mechanic analyze the engine.