Some considerations:
1. Aluminum does oxidize, just much slower than steel does (salt is a challenge though);
2. I have a 2004 Triton Alum which the steel axle is being replaced this week as a precaution ( I think had I done a better job of rinsing the underside each spring and spraying something on the steel axle, (plus 1 year my storage place left it parked on the grass all summer) I likely wouldn't consider replacing it now. Otherwise, the alum is mostly in good shape, except where any steel bolts or screws go thru (again a better rinse and anti-corrosive spray in the spring can go a long way);,
3. Krown is designed for inside channels and spaces (does swell some rubber), does drip (for days);
4. Oil - same a Krown (in my opinion)
Here is the 2004 trailer, and the old axle pictures, for you to judge, and make a call.
I think if you do a good rinse each year, and target specific areas with a store anti-corrosive spray, you should get 18+ years, like I have.
Cheers