We've got a fifth wheel, so bigger tanks than you, but we will dry camp for 5 to 7 days without moving. Our black tank is 37.5 gallons and that never gets full, but is getting closer at 7 days. And we don't make an effort to use the campground facilities. This is with a family of four.
We have 2 gray tanks, both 37.5 gallons. One is hooked to kitchen sink (which never is an issue) and the other to our bathroom sink and shower. This one's our limit. We can make 5 days work without worry, but do get darn near full, but can't make 7 days. Kids usually end up showering every other day. I've kind of figured out that our showers average 3 to 4 gallons of water each. We have had a 20-some gallon blue tote for several years and have only pulled it out to make one or two trips during a 7 day stay only to empty shower water, probably 4 times in 6 years.
We started the same way. The more you do it, the more you learn your camper, and the more comfortable you'll get. Between the tote and a few 6 gallon water jugs, we use our fifth wheel without worry. It's easier for me to carry water to pour into the camper than to overly worry about conserving water (everybody ends up stressed it seems). If we need water, I add it. If the gray fills, I empty it. For me, it keeps things easy.
You don't need a giant tote, a 10 gallon may be a good compromise between storage size and capacity. Either 2 trips in one day, or one trip every other day, would keep you in really good shape, tank wise.
My suggestion, if you are interested and have room to store, would be to get a tote, and a 6 gallon fresh water jug. Then you can add fresh water, get rid of gray water, and not be tied to full hookup sites. It will open up a lot more possibilities for places to stay.
2016 Jayco Eagle HT 29.5BHDS (ordered 12/30/15, delivered 3/8/16)
2015 F-350 crew cab, short bed, 6.7L PSD, Pullrite Superglide 3300 hitch