I believe later models all came with the transmission gauge, but I don't know the cutoff. So that is no longer a telltale sign. All Titans have come with the brake wiring in place. The plug is tucked way up under the dash above the emergency brake pedal. You have to work to get it. Is it possible that it has already been brought down and you're not seeing it?
The most important part of the tow package is the differential ratio. You want the 3.36 gears. This is usually printed on the differential itself. The door frame tag also has this but if someone switched the axle the door frame tag won't agree. The chance of someone changing the rear differential ratio on a 4x4 Titan is almost zero as they would have to change the front also.
Even at 4K your fifth wheel is well within the Titan's capability if it does not have the tow package.
The most important part of the tow package is the differential ratio. You want the 3.36 gears. This is usually printed on the differential itself. The door frame tag also has this but if someone switched the axle the door frame tag won't agree. The chance of someone changing the rear differential ratio on a 4x4 Titan is almost zero as they would have to change the front also.
Even at 4K your fifth wheel is well within the Titan's capability if it does not have the tow package.