Towing a fifth wheel with a 6" lift. Will I have issues?

Much depends on the lift type...... Suspension or body.

Some suspension lifts were know to be a show and no go as far as carrying heavy loads and keeping the trucks OEM load carrying ratings.
Because the previous owner had bags on it could mean he was carrying more weight than the 6" suspension lift suspension could handle or he may have been simply overloaded on the rear.

However for just 300 miles I would check out your route with the 13" 6" max height on fed roads and less on rural and county roads. Your call