Glad you got a good deal. Good to hear.
Downshifting from 6th to 5th. Just gotta work on it. I used to have the same trouble. Do you use the clutch or float the gears? Try reving up the engine before you try putting it in 5th and ride the gear till it slides in.
All the tires shouldn't be locking up unless you have the interaxle locked (I think).
Fuel will flow between the two tanks. At least it does on F/L's
Sounds like you just made a lousy choice in truck stops. I don't think it was the truck they were interested in and you could have looked like me and they still would have wanted to spend time with you
I have found that a tractor rides, shifts and stops better when it is towing a trailer. Truck will take less distance to stop fully loaded.
Hope this helps.