I am looking at getting another vehicle to tow behind my motorhome. I thought in the past that any manual transmission could be safely towed in neutral, but upon my 1st search, I found that not all can. It depends whether the oil slinger is on the input or output shaft.

Does anyone know if the 99 F150 manual can be towed 4 wheels down in neutral?

 

Page 167 of owners manual.
Thou shall not tow with any wheels on the road, as in your case.
A short distance at slow speeds for emergency service is likely ok.
Only possible way would be to remove one end of the drive shafts on front and rear diffs.
Good luck.