Class C, 27 feet or shorter, sleeps 8?

In 2000 they brought out "Performance Improved" heads. Those heads with "PI" molded into them, improved HP from 275 to 305 and added threads to the plug holes. If only they had machined them properly it would have been Case Closed on the plug problems. BUT some plants made some heads where the sealing surface and the threads weren't aligned, concentric, or whatever you want to call it. Not all, but enough to keep the frustration going. It took till 2004 for all the holes in all the heads from all the plants to line up. All the V10's were two-valve for a number of years, then a three-valve head design went into the F-Series trucks and F-53 Class A motorhome chassis. Those take a very skinny plug, and some of those have failed, but in a different way than early two-valve plugs.
Get an E-Series that has "Tow/Haul" on the shift lever button. That means TorqShift transmission, and by then all the plug holes were OK.