I recently experienced a U-joint failure on my 2005 Monaco Windsor. I ended up having to have a driveshaft fabricated as the old shaft did quite a bit of damage to itself flopping around under the motorhome until I got it stopped. The new shaft cost slightly over $1,000 including the parts and labor to weld the new parts together. They also balanced the drive shaft and there was a world of a difference in the vibration level with the new shaft (zero vibration) vs. the old shaft (lots toward the end of its life).
I did the removal and installation myself, but I was expecting to pay $150/hour for the mobile truck mechanic to do it (he ended up abandoning the job after one visit). The new U-joints cost around $80 each for the Chinese-made ones (the US-made ones were not in stock, or I would have put those on; they would have cost around $130 each).
I also had to source and buy a new yoke for the transmission end, new transmission output shaft seal (it was leaking a little), a new transmission cooling line (collateral damage from the driveshaft flopping around) and a new air line from the compressor to the dryer (also collateral damage).
After examining the old U-joints, the one that failed appeared to be dry of grease while the other one seemed to have adequate grease. I'm guessing that the zerks failed on the dry U-joint and the service place that I hired to do the grease job didn't care enough to let me know that (another reason to do the service work myself, which I'm doing from now on). While I was under there mucking around I went ahead and replaced the air dryer cartridge and cleaned up some zip ties that were holding wire bundles and hoses in place but were nearing end-of-life. It pays to crawl under there once in a while.