Current E450 GVWR is up to 14500. I think the change happened in 08.
Some manufactures will show you what the OCCC on the website. Some can't because the number of models and possible configurations with options will not allow that without a complex database behind a configurator program. If you can get it, the WWV (wet vehicle weight; the weight of the RV with gas, water, and LP tanks full) is helpful data point. Note that the OCCC does not include water.
My opinion: All three numbers, GVWR, OCCC, and WWV are useful. I am not sure but thought that after 2008 the OCCC is required on the passenger door. A dealer should be able to provide the WWV and maybe even axel weights for the WWV. If all the numbers are not available a part of a test drive should be a visit to a local weigh station with a full tank of water so you have a good set of data points when loading the first time. You want to fill the water tank and test the system before purchase anyway.