In our travels we have seen a lot of creativity when it comes to getting that RV level. It really is amazing what people will try putting under their tires, especially when they haven't been able to afford the latest whiz-bang technology, as much as they'd like to. Sometimes too, folks are just plain stubborn, and figure that however they can get the job done, then, well, it's done. But there are some good ways to do it and some not-so-good ways, and we've seen quite a few of them. Not to be all snobby or showing off our educatedness or anything, but there is a method to all this madness. In the end, it all has to do with basic scientific principles, something which we certainly don't expect the average RV owner to understand without some coaching. So, here goes.
Smooth, not spiny
The surface of whatever you throw under the wheels should be relatively flat, not too rough, and definitely not spiny. For example, a board with nails sticking out of it, any at all, is not a good thing to use for leveling, while a board without nails is perfectly fine, given it meets other scientific criteria revealed below. A piece of craggy rock from a nearby rock slide is not a good idea, because it is not a good match for a pliable rubber tire - in fact you could say the two are natural enemies. However, a couple of wide flat river-smoothed rocks might work just fine.
But think about what you're doing here, using heavy rocks when something a bit lighter might do the trick. You really want something that is strong but low in density, without the weight. After all, you're going to be carrying this object around with you, and you don't need the extra tonnage. Consider weight and volume carefully - a concrete block, for example, is the perfect density to use for leveling. Of course, it has other drawbacks that make it far from a perfect tool.
The problem with the big flat rock from the river and the concrete block is their size - width and length and thickness all have to be taken into account. Maybe you are thinking about one of those thin blocks that people use to build patio walls - that's fine, bit it still might be not the ideal leveler. A two by four is the first object the typical RVer turns to that looks like it might fit the bill, but it has issues as well. It's only a little under 4 inches wide, for one thing, and the lack of area coverage means it will sink into the muddy ground, along with your RV - not a good thing.
You definitely don't want anything under the wheels that will crumble, like the decorative concrete block, or be flattened, like old pizza boxes or any kind of bulky packing material. Again, carefully judge the malleability, pliability, crumble-ability, and compressibility of the stuff you're throwing under there. Realistically, anything that is easily smashed is not a great idea, and that's from a scientific viewpoint, of course.
You want materials that will last for more than one use, which rules out cardboard, pumpkins, porcupines or other road kill, and many other items that might be close at hand. Anything that will dissolve in rainwater like salt blocks is not smart, for example. Sand won't work because it tends to flow outward, and old shoes will only last for a couple of rounds under the old rubber. You probably get the picture.
Try a product designed for the job
Seriously, it's completely semi-scientific, and here's the important thing - it makes sense. Leveling blocks on the market are the perfect size, shape, density and hardness. They are smooth not spiny, and will last a long, long time. So spend just a little of that money you save by not paying a mortgage (you aren't doing that, are you?) and buy some high-quality leveling blocks. Clean out all the chunks of wood and rock, the left-over cardboard boxes and packing materials, the concrete blocks that just slow you down, and get the real thing. It will impress your fellow RVers with how smart and scientific you must be, and it will get your rig safely level.