Re: stacking block
If you mean with dual wheels, to put blocks under one tire only of the pair of duals causes all of the weight of that side of your RV to be resting on only one tire. Notice the weight rating of the tire. To block under only one tire of your duals is guaranteed to over load the tire. Just how bad it is for the tire will depend upon the length of time it is so abused and also things like weather play a part. Quite simply, doing this would invite a tire to blow out at some time in the future. It won't likely show that damage right away, but one day you will find yourself complaining about how your tires seem to blow out much sooner that for other people and wonder why.
The short answer, never put all of the weight for a pair of dual wheels all on one tire!