The salt will eat it up. No real way to get around it.
The best you can hope for is to control the rate as best you can.
Some things that help;
Before 1st use, pray the entire undersides with 50/50 mineral spirits and boiled linseed oil. Saturate the tar out of joints, connections, contact points of fastners, joints etc. Done yearly is a very big help.
Rinse the undersides when done. Some people park over a lawn sprinkler and let it run a bit.
A more radicle approach is to expose all you can for better cleaning with a power washer. That means removing inner wheel wells, underside spare tire carriers, etc. to expose the metal and anything else you can get off.
Somethings are near impossible to care for. Like the tops of gas tanks (even the plastic ones have metal fittings and sender you can't reach for rinsing), behind brake lines, rockers with drain holes only let in what you can't get out.
You can control the rate of rusting, but you'll never eliminate it all. Body mounts are terrible at rusting.
Unibodies (frameless beast) often rust worste from the inside out.
Get rid of all carpeting that'll hold salt from shoes etc.
It's a battle, you can't win, eventually you'll either have to learn to weld for repairs or trash it for anouther.
Unfortunately, it's a sacrifice we all need to acknowledge.
I sure with they allowed ATVs on the beach for fishing use only. It would make life so much easier and cheaper to maintain. Much of them are plastic bodied and aluminum / stainless. plus they'd be so much easier on the enviroment if used responsibly.
How trucks fair on the beach is totally up to how you take care of it.
If insurance wasn't do damned high, I'd have a beach only rig. you can get a older decent conditioned Cherokee for $500-$1500 off Craigs List anyday that's good enough for the beach. But the added insurance is a killer!
$0.02 from someone that's done it awhile.