You will hear from people about spitting spark plugs, it has happened to some. If you can see the front of the cylinder heads if it has a P1 stamped on it these were later heads with more threads for the spark plugs. Are you sure it's not a class A you say class C in your post. Also check to see if the hose clamps have been replaced with worm type s/s clamps. The older spring clamps they used would rust through and leak the one for the heater hose over the 1st spark plug on the right bank would leak into the spark plug hole.
If it is a class A see if they have the updated snorkel for the air filter if not driving in heavy rain the air filter can become water logged and you will loose power, no biggie but it can happen.
It's a solid engine and if it's been taken care of should go a long ways I have 130,000 plus miles on mine and it still going strong.