Here are 2 Amsoil products that will probably fill your void.
Amsoil
GWRCR-EA
Synthetic Water Resistant Grease
Resists water washout and degradation. Unparalleled protection for boat trailer wheel bearings and other components frequently exposed to water. Stays in place, lubricates and protects against rust.
Amsoil
GHDCR-EA
Synthetic Heavy-Duty Grease NLGI #2
Extreme-pressure, moly-fortified. Provides outstanding protection for steering knuckles, spring shackles, heavy equipment pivot joints and other components operating in high-load, low-speed conditions. Offers excellent water washout resistance.
Other good marine greases may work as well. I have used both of these and they are awesome. The Coast Guard uses the 1st one and they have really have minimized some of the maintenance issues they had such as what you have explained.