Thanks for all the information on the thread. I'm the friend he's asking for. Not sure it will help but what we have is a 4 star 2+1 gooseneck aluminum trailer which is just shy of 6k pounds according to the manufacturer. I figure two horses, water, food & camping supplies gets us to around 10k.
What we're looking for is a 3/4 ton or 1 ton, SRW, 4x4, 8' bed, extended or crew cab. Originally we had been thinking diesel for the mileage while towing, power if we start traveling farther through actual hills/mountains and longevity in general (currently using a '95 F250 extended cab, 8' bed, 7.3 diesel, with a manual transmission). What little we found out researching has us leaning towards 3.73 axle ratio.
I'm more open to a newer gas option than my wife but concerned about power and fuel consumption. I remember towing an older steel trailer with a gas pickup and getting about 6 mpg on basically flat land. But as repeatedly pointed out, the $9-10k premium for a diesel buys a lot of gas.
Wife leans towards Ford, I lean towards GMC/Chevy but we're mostly agnostic and want the least headache to do what we want. Not concerned about the latest bluetooth, navigation, premium sound, etc.