since yo are already at 6000 lbs with only one horse, you definitely need to move up the scale, Properly equipped 150's seem to do well as long as the weight stays down around 5k or below. Many 2 horse bumper pulls, or stock combo's especially aluminum only weigh about 2000 to 3000, but as you are finding out once you start adding the tall, extra features, dressing room, really starts adding up. Terrain can effect things as well. I plan on moving from a 2 horse aluminum gooseneck to a 3 horse with about a 6-8 foot dressing room, will add a bathroom and shower in the third stall, so will mainly be a 2 horse distance 3 horse in a pinch trailer, aluminum
I drove fords since I was 16, this last one (2007 5.4 L) I was less than impressed, long term known defects, like sparkplugs that either blow out or break off in the heads, crappy exhaust manifold studs that break, few other issues. Also the 6.0 Ford diesel is universally hated by anyone other than someone that makes their living fixing them. Yeh there are a few out there people like but reliability rating is terrible. The 6.4's were better but suck fuel like there is no tomorrow. Ford got their act together with the new 6.7, but it is so new, no used out there and they are god awful expensive. Soooooo I just bought a 2006 Dodge 2500 off road, Cummings Turbo diesel.
2006 was last year of high gas mileage, the emmision laws changed adding particle filters that killed fuel mileage, some now even require exhaust fluids to keep it clean, lots of issues with all three manufactureres. They do work but mileage suffered.
2006 Dodge got the bugs worked out of their tranny issues. So far I am pretty happy. The truck itself doesnt seem to be the quality or fit of the ford, but good lord it is easy to work on, I open the hood I see an engine, the frame to body had plenty of room that putting in gooseneck was much easier. And the power is unbelievable. I have 250 miles on this last tank and am still over 1/2 tank, will dod th emath but seems to be getting good mileage. ? I have about 20 years experience with cummings marine and generator engines so am very familiar with them, opening the hood there seems to be nothing I can't do at home, but if you want my advice if you are specifically going shopping for a tow vehicle especially one that wont need to double as a commuter, I would shop real hard for a Ford 2001-2003 7.3L diesel, or a 2005 to 2006 Dodge Cummings . I wouldnt even consider a GMC, can't say about real recent, but the late 90's are so cheap made pieces fall off them as they run down the road. Early 2000 era ones we have at work seem to stay in the shop and again seem cheap made. I will admit the newer ones seem much better but I have no experience one way or the other with them.