A lot of people here (myself included) use some form of the 'Lego Block' levellers. I use these ones (RV Leveling Blocks, Set of 10 - Four Corners D11-133 - Chocks & Levelers - Camping World
) because they often seem to be on sale when I'm in Camping World. Other's use the ones from Lynx (the orange ones).
This way, you don't have to keep long boards on hand, but instead build an overlapping stack of blocks to drive up that will be long enough for both wheels. I still keep a couple of 5' boards to put under the wheels for those rare times I'm on soft ground, but for raising a side to be level I use the blocks. Even when using the boards in soft ground I then place the blocks on top of the board on the low side to do my levelling.