Between friends, family, and workmates I know literally hundreds of people with campers, most of which tow with full tanks, and none of which have had any issues with the tanks. A couple have had frame issues, but those are the ones that tried to go ultralight with the aluminum frames and structure members. Even the low end springdale TT I had a couple years ago had enough carrying capacity to where I was almost 500 lbs under my max when loaded for camping to include a full load of water. Again, how are you going to boondock, or camp in backcountry areas if you don't have water? When it's time to leave are you just going to dump your tanks on the ground as a present for the next people? As for filling all the tanks, for my type camping it doesn't work out quite that way. For the most part I just end up transferring from the fresh to the grey and black over the course of the trip. On longer trips I do sometimes take 35 or so extra gallons of water in the back of the truck as my freshwater tank only holds 50 gallons while the greys hold 80. We don't do that often though as we tend to move spots after 4 to 5 days.
The advice to check you tank mounts, along with virtually everything else on your rig on a regular basis is a very good idea. No matter the brand, these things a pretty much just thrown together and you'll find stuff needing tightened, re-routed or repaired. Things also work loose over time, especially if you travel a lot of miles over rough roads.
