I installed a Progressive Industries hard wired unit into our 5er and couldn't be happier. Since installation it has cut power due to low voltage, and in one case badly wired post. If something goes wrong it tells you (through a code) what the problem is. This allows you to address the issue with the campground management. I have done so and it takes all of the guesswork and speculation out of the equation.
The company is extremely customer oriented and will help you through any issues or questions you may have. I have a story in this arena but won't recount it now.
While it is a bit more expensive than the portable units, the surge protection is better and you won't have to worry about getting it ripped off. Here is the website:
http://www.progressiveindustries.net/ems_hw30c.htm