Does your trailer sit outside? If so I think the sun is the reason for the cracking.
I have used turtle wax for years. Mine aren't cracked, but, my trailer has a building around it when not in use.
Some of the guys use Protectant 303 on their decals and swear by it. It's suppose have a UV blocker in it.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow
I think a building around your rig is a good investment. These things really don't stand up to the weather very well. As for waxing your decals, I've done it with every trailer I've owned and had no detrimental effects, but I've also kept them inside when not in use.
Mike in Tennessee
2007 Cruiser 30QB
2008 Ford F250 Lariat 4x4 6.4 Powerstroke
Valley 16K slider
Ride-Rite air bags