CAG brings up good points. I have had a 35 gallon bladder I got from Camping World years ago. It was made by a company in CA, though I don't recall the name. I think CW still carries them. Mine has lasted extremely well for a very long time. I have been impressed that it is very easy to dry after use and I have never had any mold inside it.
Just be sure to buy something that is confirmed for use with potable water.
When boondocking, I can almost fill my fresh water tank, when empty, with the bladder. I would have a hard time fitting a 100 gallon bladder in my truck bed with my fifth wheel hitch in place, and I wouldn't want to put much of the bladder on the tailgate. You also have to consider the "roll" effect while driving any distance. I built a wood platform with sides to keep mind under control for longer distances.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS, Progressive Industries hardwired 50A EMS HW-50C)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels