After towing a 5th wheel trailer on a number of long trips, I'd share some basics on what to look for in weight and safety. To the dry weight of the RV, add the weight of all the tanks when full (black water, grey water, and propane). This will give you a real world idea of the weight before you start loading in personal items that you think are necessities. With time you'll find that about half of the items you thought were necessities were not needed. I add 1000 lbs for personal items for the occupants and food. Slides provide a lot of extra room, which is nice, but really add a lot of weight.
Plan on going up a load range on tires, because RV manufactures seem to use tires that are marginal for the weight of a fully loaded RV. Blow outs cause some real safety issues and cause RV damage. Go to a truck scale to actually see what kind of weight you are carrying on the front and rear axles of the tow vehicle and on each axle of the RV. Use a tire inflation chart to determine how much air to carrying in tires. Do some thorough research on RV construction materials--aluminum vs. wood frame, different skin materials, and roof materials. Fuel economy, IMHO, is based as much or more on wind resistance than the weight you are pulling. Figure on getting 50% mpg less when pulling a heavy RV than what you experience when running the truck unloaded at highway speed. Being able to safely stop a heavy trailer is more important than being able to pull it. An exhaust brake is a necessity, and the best heavy trailer brake on the market is also a necessity (MaxBrake). A transmission temp gauge, and an EGT gauge are also necessities. Sorry about the long post, but I'm pretty opinionated on this issue. Try short trips first to get a feel for stopping, starting, parking, and driving in traffic. Be safe, and have a great time!