I find it incredible that 4 people managed to fill a 38 gallon black tank in one night... The wife and I can go 4 days easy on our 40 gallon tank.
Back to your question though, most of the foot pedal Thetfords don't have a control for the amount of water; the longer you stand on the pedal, the more water it uses, but, even then, I still find it incredible that you could you generate 38 gallons in a night, that's 9.5 gallons per person. A 1.5 gallon household toilet (and a Thetford uses far less than 1.5 gallons) with 4 people going an extraordinary 5 times each in a night would only be 30 gallons....
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)