Yes it's okay to take an older vehicle with high mileage on a road trip.
As long as the high mileage vehicle has been maintained and you've changed the oil, checked the fluids, belts, tires etc then the high mileage vehicle should make a road trip just fine.
The long road trip will be good for the high mileage vehicle because it helps to bring the engine up to temperature and gets the fluids going through the vehicles engine and other systems.
I have a truck that has 500,000 miles on it and I do not hesitate to take it on a long road trip.
I do have extra money in case I need a tow but hopefully I will not need it.
I also maintain my vehicles and make sure I have good tires, fresh oil, fresh transmission fluid and make sure to top off those fluids before and during the road trip if needed.
Every time I stop for gasoline I check the transmission fluid and oil just to be sure it has not dropped or I have not gotten a leak somehow.