They are good, I've used them before most people knew about them. On my higher mileage vehicles I did see an increase in tire life.
These will not only keep your rims/tires in balance but the entire rotating assembly.
Here is where you need to consider whether or not you need them on a Motorhome. How many miles do you put on your motorhome annually? If you don't put a lot of miles on then it's probably not going to help you in the tire life department. Your tires will Age Out long before you wear the tread out.
These won't fix Alignment issues. If your king pins (or ball joints) or other steering parts are worn out or misaligned you will wear out your tire prematurely anyways. If this is your problem spend the money on fixing your suspension and alignment.
If you are trying to create a smoother driving experience by using these I highly doubt you will see much improvement unless you have something fundamentally wrong with your rig. Again I think having a chassis shop fix the issue is money better spent.
Some people just buy these to avoid having to have their wheels rebalanced in the future. Unless you live on the dark side of the moon I personally can't see how this is an issue. But to each their own.
Only you can decide if these are for you. I think they are a great product so nothing wrong in that department.
As for sizing, just take it to the tire shop or call or visit Centramatic's website.