Paul,
You get get the back off with an old frying pan. Put pan on the stove- med high heat - let it get to temp and place the tank rear cap down. A hot glove to grap it with and a flat screwrive to help hold the cap down and when the solder melts you just pop the end cap off.
Clean / inspect / clean outside, when ready to put cap back on, heat up th pan. have some solder ready, place the end cap on the tank, put it in the pan and pres down so it makes good contact.
Wait a minute and touch the solder to the edge of the end cap, if it melts and flows in a few seconds continue around the edge, if it blobs, let the tank heat a bit more and try again. If the solder in the body seam melts and flows out, give that one a bit of solder too.
Pressure check when done, if it leaks - try again, or get a small propane / butane torch and use that for more localized heat.