I wouldn't, especially if you'd have to use plastic caps because they can crack. When they crack and your oil drains out, you won't necessarily know about it until your hubs are welded to the spindles on the highway. With grease, you can lose the cap entirely and go hundreds of miles with zero issues and deal with it when you have time.

For the kinds of travel I do, I prioritize reliability and durability. Your situation may be different.

 

I agree with @jdwarren and especially so for a boat trailer that will likely sit unused for long periods of time, oil seals don’t like that.

 

Vault hubs. Flawless. And I tow my boat ~5000 miles/yr.

The VAULT Hybrid Lubrication System

The VAULT “Hybrid Lubrication System” uses a semi-fluid oil in a pressurized chamber to give you the ultimate in wheel bearing lubrication and protection. At ambient temperatures, the oil is thick with a viscosity approaching grease. As the bearing temperature rises during towing, the VAULT’s...

pacifictrailers.com