my engine overheat, please help

I am traveling with a van. The engine overheat, especially when climbing slope. If driven on a flat road, it looks OK.
I already added coolant, actually there is no coolant leakage.
I need to drive another 400 miles to home. I might not be able to have it fixed locally since tomorrow is Christmas holiday, shop is usually closed.
Any suggestions that how I can drive safely to home without engine broken?

Min