That length is more likely to have no more than a 50-60 gallon capacity. It's easy enough to carry extra 5-7 gallon jugs of water in a storage bin though. The on board pump can be set up to draw it out the jugs to replenish the main tank, if the RV isn't already equipped that way.
Dutch
2001 GBM Landau 34' Class A
F53 chassis, Triton V10, TST TPMS
Bigfoot Automatic Leveling System
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox baseplate