High Water Temperature and Engine light on

The short answer is that it means you may have a serious problem that should be checked out by a competent mechanic before you attempt to drive somewhere else.

It could be a simple fix, but it also could mean that your engine is about to be junk, if it isn't already. All those things suggested need to be examined and ruled out or repaired. If you overheat an engine, you can do serious damage.

I would suspect your fan clutch if radiator is full.

I assume that during your 100 mile drive that you didn't have any water temperature problems since you mentioned that this happened right after you finished. It is not too unusual for a temperature to spike on an engine that was run hard and then brought to an idle while stopped. Most of your cooling air flow has stopped and sometimes the temps go up for a short time. Of course your electric/hydraulic fan could also be bad which you might not notice when driving.
As mentioned, in addition to a cooling system flush, it would be a good idea to check your radiator for air flow restriction caused by debris in the exterior fins as well as corrosion of the fins.

20 year old coach may be due for a new radiator. They tend to plug up over that period of time if they have not had regular flushing and coolant changes...

Low on water....broken belt...low on anti-freeze.....may I add a bad water pump?

How about a bad sending unit? All kinds of things.......................

It probably means you need to add anti freeze to the radiator.

What did the water temp gauge read? IF the temp on gauge is over 220 it means you have a cooling problem... low on water or broken belt.