Before you spend the money on a whole house filter look up the specifications of the ones you're interested in. I think you'll find some of them don't filter out much but sediment. Just like the one under the sink the more it filters the more expensive the replacement cartridges.
We looked at a whole house system a couple times, but decided against it. The ones that filtered the same things as the under counter system cost an arm and a leg for replacement cartridges. To top it off you would be filtering more water for the shower and toilet than for drinking or cooking.
Secondly any water system used in a commercial (i.e. campground) setting is subject to at least annual testing and certification that it meets standards for public consumption.
Here's a link to the type of filter originally installed in our 2013 Adventurer
3M Under Sink Filtration, Replacement Filter, "A"
As you noted it doesn't filter out much. That's probably why the price is only $27.17
Here's a link to the one we used to replace the original. It's a direct fit and filters out much more, but then again it costs more at $38.00
3M Under Sink Filtration, Replacement Filter, "C"