Loaded Question: "How easy is it...?"

Replacing it shouldn't be beyond doing yourself - if you have good access to the area and are careful each step of the the way. Keep in mind that it's "plumbing", and as such, making the install leakproof for years means thinking a lot and cutting no corners on your workmanship.

I addition to the replacement tank you've located, here's a website showing a few more fresh water tank choices:

http://www.plastic-mart.com/category/33/rv-water-tanks