It sounds like you're looking for something like the RV City Water Inlet w/ Brass Check Valve # 37262115. This has the check valve built-in so you'd need to use it as a complete replacement part.
The cutout needs to have a 3-1/2" diameter and then the flanges have a 4-5/8" diameter so you need to have that space on your Forest River.
If you're looking for something else just let me know! I've add a review video of this part for you to check out. view full answer...