You should use blocks on both wheels to prevent too much weight on one tire. Also you must use a large enough block(s) to provide support across the entire tread foot print. If you use too small of blocks you can damage the tread or bands of the tire and will cause premature failure. Usually tread separation.
I use 24 in. long 2X4s blocks layed out side by side (usually four or 5) which provides full coverage of the tread foot print on my duals. If I need more height I stack the 2X4s log cabin style but the top row is always perpendicular to the the RV.