If you are going to pull a Toad get a big block gasser like the Ford V-10 or Chevy V-8. The Mercedes makes a poor choice for towing. Most newer E-350/450's are rated to tow up to 5,000#, don't settle for less. Be sure you get the large fuel tank, 55 gallons works well, 40 will be a Pita. Expect 8-10 mpg as more than that just is not going to happen.
As noted you want a decent CCC, nothing less than 2,000# but preferably more. A fiberglass roof is the way to go, one piece fiberglass front and rear end caps are also advised. Slides can create as many problems as they allegedly solve, some love them some do not. Look for a floorplan that works with how you will use the coach the most. AC and heating systems need to be appropriately sized for the climates you will be in, many are insufficient for all but the mildest weather.
Check the mattress where you will be sleeping, it should be a quality innerspring. Lots of headroom, 6.5' minimum will be much more comfortable in the long run. You cannot have too much storage both inside and out.
