This is a typical WFCO Power Distribution Center where all of your 120VAC Circuit breakers and 12VDC fuses are located. Most of the time the module in the bottom compartment is your CONVERTER. The Power Distribution center is usally close to the fridge or oven and mounted close to the floor like this one shown on google search.

Inside this WFCO model you can see the circuit breakers and fuses


With the inside panel removed you can see the most usual place where the converter unit is installed in the bottom compartment

Keep in mind that there are many different companies that produce the power distribution centers and converters. Just showing a popular WFCO brand here. Also Motorhomes are alot different than regular RV trailers where things are located but the basic electrical design is much the same. The older units like yours can be really unique... WFCO may not have been a company in 1977...
The INVERTER is customer optional installed in most cases and IS NOT your converter. They perform two different functions.
You should read up on how RV's work online- Just type in 'HOW DO RV's WORK' into google and go from there...
Roy Ken