I actually added a 20 gallon tank on a 24 foot C with a 450 chassis years ago.
I suspended it between the chassis rails as others have described, installed a fill hose and a discharge hose and then used a small pump to transfer to my rig's regular on board water tank as needed. This was for boondocking and it worked fine.
I always filled the tank near the campsite. With that knowledge, I would say those that suggested the hitch carrier have a better idea. Install a hitch carrier, strap on an appropriate sized water tank and you would be good to go. I certainly wouldn't fill the tank until I got to the campsite area and I would probably limit the tank size to no more than 25 gallons max. I had rather be safe than sorry.
Also, when traveling and not boondocking, I would just leave the hitch and tank at home and be two feet shorter and have less to mess with.
