Do you need the entire housing? ...or just the filter?
Winnebago used the Shurflo/Pentek housing and is readily available at retailers. You don't have to order it from Winnebago nor is it a proprietary Winnebago part.
If you need the housing, they're less than $50 and I can help you shop for it if you'd like. It should look similar to this:
SHURFLO 158196 Housing Pump
Notice how it lists your Winnebago part number.
If you just need the filter cartridges, you can buy those anywhere --even at the big-box stores, see below). They're a standard 2.5" x 10" (or sometimes described as 2 1/2" x 9 3/4") Pentek-type carbon 5-micron cartridge.
If you need a cartridge immediately, both Lowes and Home Depot will carry one that will be compatible. Again, they are just generic cartridges and will be labeled with different manufacturers. If you get on Amazon, you can buy them by the bulk and save money. You can buy the standard-level Shurflo but they'll cost a bit more but I don't think there's any substantial difference. Or, if you want to upgrade above what Winnebago supplied as OEM, you can purchase a more fancy filter cartridges that will fit.
We just used the standard ones and changed them often as we didn't usually drink from the incoming water.
Here's the one I bought when I needed one at retail ...I went to Lowes and bought either a Whirlpool or AO Smith 2-pack depending on what was in stock:
AO Smith at Lowes
Whirlpool at Lowes
Home Depot will have a GE 2-pack:
GE at Home Depot
edited to add: Now that I re-read your post, it sounds as if you need the housing itself and not the filter. I just looked it up at one of my favorite online retailers, RVUpgrades.com (I recommend them not just because they're a forum sponsor and the owner is the founder of iRV2, but because of the excellent service and fast shipping), and they have it for $20.64 but it appears they're currently out of stock. Also, if you look at the same page linked below, RVupgrades sells the actual Shurflow cartridge for $6.25 so I was wrong about it being more expensive.
Shurflo 158196 Housing 1/2" Female Ports - 10"
If you need it immediately, it looks as if other online vendors do have the housing available. Let me know and I'll give you a link.
I believe the housing includes one o-ring seal but check with retailer to make sure. If not or you need an extra one, I just noticed this on e-bay (It's Shurflo part #9417809)
SHURflo 9417809 O-Ring Kit 2 Pack