There are differences between a 2004 and a 2006. The 06 is higher inside. Has a nicer rear cap and some other changes. All of those units will have the Cat C7 but the 07 might have a dpf. If it does do not buy that unit. cat never did figure out the emission system and their DPF has lots of problems. The 06 will be DPF free. Bounders give you a lot of bang for the buck. The cabinetry is one of their weak points. Vinyl wrap that is very thin. The inner walls ( like the bathroom) can be flimsy. But overall most owners are very happy. We had an 02 and an 05. Loved them both. Also very easy to work on. Unlike newer units not too many computer boards. The 05 still used Power Gear jacks and slide outs. Good units. Many newer units may have electric jacks and those darn Lippert slide outs. FW has amazing tech support . Even for older units. The Cat fuel injection pump is another weakness as is the fan hub bearing.
Moisheh