If the engine is at operating temperature, and both heater hoses are hot - as in 'difficult to hold onto for very long' - would suspect the temperature-blend door is not swapping over or sealing off completely.
One check may be to move the temperature selector from hot-cold-hot several times to see if there are any changes or any unusual noises like thumping or popping. Wondering if it may be on the edge of sticking or binding - as you said it did get hot once.
May have to bite-the-bullet and lay some eyes on the blend door actuator. Not for sure on your model year if there's an access cover to remove to be able to stick your hand up in there to feel around.
If you see your temperature gauge making some unusually wild swings - a typical sign that there may be air still in the system.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 3K miles.