I have a 2015 cardinal and it came with the kw1 / fr1 canister and filter cartridge. i think yours probably has the same but go out and look at it to be sure. if you do a search on these keywords (fr1, kw1) you will find many post and discussions about them. these are apparently proprietary to forest river and the consensus is that they are expensive. some people like them, some don't. i know they are made by waterpure but are available via many web site suppliers. i found that ordering replacement cartridges on-line was far faster than ordering via a rv dealer. one thing that should be mentioned is that they also make a plastic wrench that aids in removing the canister. it is only a few dollars. there are many posts about the little tabs on the canister getting broken.
2015 cardinal model 3825fl
2015 dodge ram 3500 dually