You may be able to identify the model you have by using the Winnebago brochures at Previous Model Year Product Information
Enter the year of your rig.
VIN number on a title may or may not be based on the chassis VIN. Winnebago builds the house on a mfg chassis (Dodge, GM, John Deere, etc.) Dodge (Fargo) VIN is the one you need for chassis parts. Dodge ID plate is usually under driver window
or in the radiator service area (open radiator access grill in front of rig) on drivers side.
Most Winnebagos were built on the previous year chassis so, a 73 would most likely be on a 72 chassis. 27Ft model is most likely on a M375 chassis (M400 [RM350]) and would typically have a 413. 318 would be too small for that chassis. 413 has the spark plugs on top of the exhaust manifold. 440-3 (first installed in 73 chassis) has the spark plugs under the exhaust manifold.
For a class A rig, you are going to need both the Dodge MH service (69-73) and parts (69-77) manual. You do not get these through Winnebago.
There is lots of information out there. Search the web for Classic Winnebagos.
The "Eyebrow" Winnebago Coach Service Manual covers 73 and earlier rigs but most of the information applies up through 78. Available through Classic Winnebagos & Vintage RVs!
Most house parts (depends on item) are mostly a junkyard find or self built.