I tested mine with and without the water filter. At least with a clean filter the difference was minimal for me. I do both boondock camping as well as camp hooked up, so I use both filters when I'm hooked up. That way I always have the on board one when boondocking. The hose filter takes care of really bad tasting campground water, and doesn't clog up my on board one. There have been a couple campgrounds with pressure so low that water only trickles out of the shower...for that I usually disconnect the hose filter.
Also, the canister filters are much cheaper if you get them here http://www.rvwaterfilterstore.com/Pumps.htm
As for your water heater, it's not really adjustable. Mine puts out 120 degrees easy. Did you let it rung long enough? The light on the switch only indicates ignition time, or if there is a problem with ignition. The light goes out while the heater is lit and still heating. It takes mine around 20-30 minutes to get hot depending on ambient and water temperatures...
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