The value of a 5th wheel vs. TT is highly subjective. 5th wheel RVs cost more in order to get the same features and size. But 5th wheels tend to have a lot more storage and more headroom in living area, as well as are offered with a lot higher end conveniences and features.
As for RVing for your reletive's truck, a TT will offer far more options than a 5th wheel. I think the tow rating is likely higher, but it probably doesn't matter. Is is quite easy to exceed some other rating with a 7000 pound loaded TT, let alone a 5th wheel of same weight.
Let's take a typical half ton truck with 1500 pounds of payload. Some have more, some have less. Your relative can check his Tire and Loading sticker for his exact configuration as equipped. Let's say he needs 500 pounds of that payload for the family, and 200 pounds of camping gear in the bed. I've never seen a pickup truck pull into a campground with an empty bed. So there is 800 pounds left for trailer tongue weight. That's abut 6500 pounds loaded TT. With Ford's recommendations of restoring 50% of lost front axle weight with a WDH, owners tend to report that also puts them right at rear axle rating as well.
Now the 5th wheel example for the same truck. Starting with 800 pounds of payload left after people and cargo, you need a 150-250 pound receiver, leaving 600 pounds for RV pin weight. I think
this Scamp will even exceed that when loaded up.
TTs can tow just as well as 5th wheel RVs. That means a $2000-3000 WDH though. Most people are pleased with the towing comfort provided by a ~$500 WDH with integrated sway control like the Equal-i-zer 4pt, Reese Strait Line, Blueox Swaypro.
As for water - I always fill up my fresh tank before leaving home. Most of our camping is electric hookups only, so it saves some time at the campground filling up with the family anxious to get out of the truck. It also means we can use some while traveling for emergency potty breaks and washing up.
