How likely am I to need 30amp and/or water extensions?

Thunder Mountain wrote:
If you don't have them, you will need them. If you have them, you will still need them.

The real bummer is when you are 3' short and don't have it.
It is worth the expense to be sure you have everything you need before you come up short. You know the prior planning thing!
Waiting until you absolutely need the extension/adapter won't make the item any cheaper but it will be a lot more inconvenient to track one down.