My surge tank on my 2000 Safari, Sahara was also sun degraded. However with a little research I was able to find a Ford replacement that is an upgrade from the ones purchased by Monaco. (all plastic)
Ford part #F6HZ-8A080-B is the exact same part except has a metal fill neck and an improved plastic construction.
I found it on Amazon.com for $144.53 with free shipping from a third party seller "Tasco Automotive Group" (the old style from Monaco is now $335.00 plus $35.00 shipping)
I keep my Safari (previously own and stored outside) parked inside my RV Garage so the sun issue should not be a problem in the future.