My old TT had zero rust after 4 years of storing the stinky slinky in there.
My new MH came with a bit of rust inside the bumper. I read a post on here a few years ago where a guy actually painted the INSIDE of his bumper with POR-15. If the rust gets any worse, I may employ that tactic.
The PVC fence posts are a good way to go, and definitely worth consideration.
I prefer to store in the bumper. I put caps on both ends of the hose so insects and water cannot get in or out of the hose itself. This also will help keep the hose inside the bumper in the event I lose an end cap in transit. I condense the hose as I'm pushing it into the bumper, and then place on the hose cap, and then push it 8-12" further into the bumper. The caps seal the hose and don't allow it to expand freely, so if I lose a bumper cap, my hose isn't going anywhere. So my risk of corrosion comes from sources OTHER than the stinky slinky. And my insect risk is high because I have not screened off the openings in the end caps yet. I keep the ancillary fittings in a container in a storage bin.
-2018 Greyhawk 29MV
-1994 Jeep Wrangler YJ (Toad)
-2015 Jeep Grand Cherokee Limited (Soon to be Toad)
-2014 Jay Flight 28BHBE & Ram 2500 6.4L CC 4x4 (sold)