I use ramps cut from PT 2x10s, and they haven't shown any great signs of cracking/checking. It probably depends some on the exact wood and how well it's been seasoned and so forth how well they hold up.
If you use, say, a couple of 1x6 planks side by side and laminate exterior (I'd probably use pressure treated) plywood on the top and the bottom they should last a lot longer. Even if the internal 1x lumber cracks it will still support weight in compression just fine, and the plywood will keep the pieces in position. They'd also be less likely to warp if constructed this way.
Since I don't have any other means of leveling, there's no chance of my having a motorhome wheel off the ground. I do have a driveshaft parking brake, so having a back wheel suspended for any reason without suitably chocking the motorhome is not at all a wise idea for me.