If the specifications say 29' 8" long, that is a measurement bumper to bumper, and may not include add-ons like spare tire mounts, hitch receivers, ladders even if standard equipment.
If they say width 100" or 101" or 102", that is outer skin to outer skin, not including awning parts, mirrors, door handles, latches, clearance lights, and maybe not even vents and covers for appliances.
If you are talking about a number in a model designation like "29B" that is not an advertised measurement of anything, it is simply a model designation. You will only confuse yourself if you try to interpret that designation to be a description of the RV, since different manufacturers make up those names in different ways.
How much of the overall length will be house, how much is cab, depends on the cab chassis used. Ford, Chevrolet, Dodge, VW, M-B Sprinter cabs are all different lengths. The length of Ford, Chevy and Dodge changed over the years but have been stable for a long time, last Dodge was unchanged for something like 20 years. The Sprinter has changed cab length since it's introduction to N. America.