454 Exhust Headers
I am guessing that you mean the cast iron manifolds, rather than the high performance headers? If you do mean manifolds, the cast iron ones that are standard for both the 454 and the Ford 460 are famous for cracking due to the extream high heat that they must endure. I would suggest that if your manifolds are cracked, rather than replacing them, consider a set of headers. Headers will not only eliminat that problem completely, they also greatly improve performance. Banks has the best quality kit for this, but they also cost the most. If you plan to keep the RV for a long time, then I would reccoment a Banks. But if not, then look to either Gibson or to Thorley. All three of these companies make what you need to solve the problem and at the same time, improve performance.