4 wheel drive class c

Gary, really like your rig,I have a 28 ft c on an e350 with extensive water damage . Upon the rebuild I plan on wacking some off the back to bring it down to 24 ft. for years now have been researching a4x4 conversion and u joint would be my choice, one hang up has been keeping within my gvwr of 12500 as the conversion would add about 800 lbs. Any idea what your rig weighs comfortably loaded with full tank of fuel? Thanks .