Those are some great questions, and I am glad you asked!
It shows me that you are concerned enough with your set to take care of it.
""Is there any way to test the fuel pump before I go through the task of removing the genset from the coach? ""
There are actually 2 ways to test the pump.
1. Run the set until it quits.
Remove the fuel line from the carburetor and place it into a clear empty container - an empty soda bottle works great.
Press and hold the 'Start' button for about 5-8 seconds.
There should be a steady, pulsing stream of fuel. If not, the pump is bad.
2. Obtain a low pressure test guage.
Run the set until it quits.
Remove the fuel line from the carb, and install the guage to the fuel line.
Press and hold 'Start' for a few seconds.
If the pump is good, you will see a pressure of 3-5 psi, and it will hold for at least 10-15 seconds after you let off the 'Start' button. If the pressure is less than 3, or if the pressure does not hold, the pump is bad.
""I have accessed the top of it already, and can remove the fuel feed line from the gennie in order to test the flow there (blew into it and heard bubbling in the coach's gas tank).""
You should never back blow into the fuel system. Doing so at the genset can blow the pump, as it is a low pressure, diaphragm pump.
Doing so from the genset back into the tank can also do damage.
Some pickup tubes have a very low pressure check valve in the tank, and you can easily destroy the check valve. This would not damage the genset or fuel tank, but what it can do is make your set take longer to start.
This check valve (when present) is there to hold fuel in the line, so it does not all flow back into the tank. Without this check valve, when you go to start the set, it will have to crank until it brings fuel all the way back up to the set from the tank.
""Have you heard of a fuel pump that fails only when it is warm?""
I sure have - and I see it all the time. One of two things can happen.
1. The windings in the pump motor get weak. When they are cold, it works just fine. But when they get hot, usually after 15-20 minutes of running, the wiring starts breaking down.
2. The diaphragm is made of rubber. If it has a pinhole in it, it can work mediocre until it starts stretching. Then it will not work at all.
""As for your last suggestion - the genset has less than 400 hours on it - we exercise it for about an hour each month, and it has run well for us until this last trip, where it suddenly failed. Is it worth taking the time / expense to do the brushes, rings, etc. with such low hours? And if so, where do I find info on how to do all of those things?""
If you are exercising it regularly, it should not be that big of a problem. It is when they sit for long periods that the damage sets in.
The slip rings are made of copper, and as you well know, copper oxidizes.
However, running the set keeps the slip rings clean as they spin against the brushes (which are made of carbon).
For the information on how to do the rest, you could purchase a service manual for the set (I think they run about $40), or I could walk you through it.
""And any idea what all of that would cost?""
No, but I can give you an idea.
Assuming the set is already out of the van, to disassemble the set and service it would take about 4-5 hours. Whatever the prevailing labor rate in your area is, I have not a clue.
But the national average at an RV shop is between $90-$100 an hour.
If you have been exercising the set regularly (it's rare for people to do that), there would be no reason to suspect the slip rings or brushes are in need of servicing, with only 400 hours on the set.
I just usually suggest it, as most people don't exercise their sets like they should, and it would be bad to remove the set to replace a fuel pump - only to have the set quit 2 weeks later because they did not spend the couple extra hours on it while they had it out.
I guess I should also warn you, that if you decide to tear it down, a couple of special tools are required, like an impact wrench and a steering wheel puller.
You would also need to either get some special bolts to install the puller, or have a drill and tap set, as the flywheel is not tapped to accept the puller bolts.