That's a huge question. There is a ton of info over on the Escapee's forum. They have a special section for HDT's (Heavy Duty Truck) here: HDT - Escapees Discussion Forum
You should read closely the resource guide and the faq thread. Lots of people are doing what you want to do. Also consider attending the HDT Rally: HDT Rally
I've been there 4 times and have a great time.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio