That's a very good deal!  Just for the router, it's a good deal.  And since you are already on Sprint, it's even better although the offer says it requires a NEW 2-year contract with Sprint.  My only caution would be that, while Sprint has lots of satisfied RV customers, I believe most full-timers have Verizon as the overall national coverage is a little better.

We have the same router and it works quite well.  It is all you will need to be able to have your own in-house "Wi-Fi" so that you can share the cellular internet connection between computers assuming, of course, that you always have a cellular data signal.

Now, do you need additional equipment?  It depends. 

Your internet connection will only be as good as your cellular signal wherever you are.  As you move about the country and get away from populated areas and interstates, you will find weaker and weaker signals and sometimes no signals.

If internet is something you need or want every day, no matter where you go, you can increase your chances in rural areas by adding an "antenna" to the aircard.  That Sprint air card in the promotion does have an external antenna port and 3G Store does have antenna options for that air card.  I think 3G Store would recommend, at a minimum, the addition of an antenna.

In fact, they have a page called "RV Solutions" where they lay out "Simple", "Good", "Better", and "Best" solutions for RVers.  All of those options include an antenna.

The "Better" & "Best" solutions also throw in an amplifier.  An antenna improves your coverage and increases your initial cost and an amplifier boosts that coverage even more, but comes at an even higher initial cost.

So, it's all about balancing the overall cost with your desire to have internet as much as possible with the areas of the country where you want to park. 

We like rural areas and we find ourselves without cellular service or with weak cellular service all the time.  For example, where we are parked right now, we waffle between 0 and 1 bar on our cell phone with 1X service (the 1X simply means it's the oldest technology and slowest data signal we can get).  However, with the antenna and amplifier, we increase our signal to three bars.  Whereas we had dropped calls when we arrived, after I set up the antenna and amplifier, we've had no problems.

So, with the money you will be saving on the air card and router, consider adding an antenna and/or amplifier IF internet is really important and you plan to be out in the boonies from time to time.  Still no guarantees, but you improve your chances quite a bit.