I started reading from bottom. It appears that you have a '75 alright, but as it is clearly not a GMC upfit we may have trouble answering some questions. This is not for any reason other than your coach was not fit out as many others, but we will try to help none the less.
Speaking of help, I noticed but forgot to mention that it appears that your read ride height controls have been disabled. This is something that you should look into if you plan to travel much. The rear load can vary a lot from full fuel and water to empty and the handling will be affected. It is also nice to be able to level up (essential with an absorption reefer) when you are parked. It may take some effort, but I believe it to be worth the work.
Now, this is a community like few others. It is most like the watermen of my world. For that reason and even without a name, I like to welcome new owners much as you might be there. So,
May the Good Lord bless this coach and all those that set forth within here.
Welcome Racemech?
Matt