my 2012 fleetwood motorhome ford v10 keeps losing coolant from bottle. on 2012 Ford Transit Connect

it loses coolant when traveling @ interstate speed. was pressure tested at ford dealership they foungd no leak. the coach has 8800 miles and we have add 5 gallons of coolant.the leak has gotten worse. do you think it can be leaking from tattle tail on water pump due to it turning faster at higher rpms. you can let it ideal for hours and it does not leak.. or maybe a faulty radiator cap....coolant leaking on frame & top of engine box...

should be under warranty still but poss cracked coolant bottle. very common
2 more answers
Slight leakage is hard to find, sometimes even bigger and large discharge. Such as table there is a heater tank, sometimes slightly leakage is difficult to find sleep. Such as cylinder pad or cylinder internal mild water seepage is hard to find. Or patient slowly find leakage, or that first open, and serious to find, easy to some. The lid of the water changed yet? Sometimes the things slight leakage, a very cheap. Try
this web you can
This is a complex problem, I suggest you to have a look at and find some useful car tool for you.