To replace the water tank, you have 2 water lines in the back that must be disconnected. You have a propane line, and the wiring for electric ignition. Most of them come out through the side wall, but you will need to access the rear to disconnect the plumbing.
Now if you change brands or models, things may not hook back up, and you will have to do some modifications, plumbing, gas, electric. Whatever you do, once replaced, make sure you turn the gas on, do not light the water heater. Use soapy water to check for leaks on all the connections you had apart. Not too bad a job if you are somewhat handy with tools, and don't have to do any modifications.
Take pictures as you go, and if you get lost, post pics and problems, and someone will help you out.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT