I’d bet on the boat launching scenario too - tons of trucks up here are rusted on the RR axle shells but spotless elsewhere.
Why I went back to the biggest donuts I could as esp one launch is retardedly flat - I’ve seen small trucks have to back out so far the guy opens his door & steps in ~4-6” of lapping saltwater just to float whatever he/she has off the trailer.
I’d check the bearing grease too - just to be safe.
There’s a reason oil-bath boat axle bearings are the standard over old school grease - FWIW, late 90’s oil-bath on trailers became the new norm.