The empty weight is easy - any vehicle scale an get that for you. Cost should be $10 or less if you don't ask for a "certified" weight. Many trucks stops have scales, as do feed mills, sand & gravel yards, moving van companies, etc. They all sell stuff by weight, so usually have scales available.
If you can identify the chassis (Ford, Chevy, Dodge) under the coach and the wheel base, we can probably make a good guess on the GVWR (max gross weight). Or get the VIN and decode it - the GVWR is usually coded in it somewhere.
A 1994 vintage gas chassis motorhome is typically on a 12,000-17,000 lb GVWR chassis. I doubt if a 28 footer is at the top end, so you are probably in the 12000-14000 range.