Hello, I am new to this forum, and I've browsed a bit. Seems there are a lot of knowledgeable people here, which is a good thing, because I'm stumped!
Well, where to begin. My house was built around 1915, and is on the side of a hill. There might be a crawlspace underneath the basement floor, but I would have to tear up the floor to find out. The problem I am having is with a sewage odor. When it happens, which is very often, it is throughout the whole house, and about 3/4 of those times, you can also smell it outside. I have municipal water and sewage at my home, and the borough won't do anything about it. I don't know what to do. There are no leaks inside the home, and the sewage drain line runs out under my front yard to the street, through the basement wall. What do I do?

When I moved into my house 4 years ago I had the same problem.
I identified it immediately as a improperly seated toilet wax ring.

Old houses like yours and mine have had their share of weekend warrier type people working on the plumbing, its been 4 years and Im still getting things straightend out. Anyway, here are some things to check on:

If its out in the yard as well, you may have a sewer line leak somewhere in the yard, or close to the house. Check (sniff) all of the drains in your house, also check around the base of the toilet for strong odor.

If you notice the smell strong by the base of the toilet, pull the toilet and inspect/replace the wax ring. If you pull up the toilet, have a new wax ring handy, you can not reuse the old one. They cost about $3-$5 at a local hardware store.

Check all drains, if there is odor coming from the drains, pour water in the drain. If the odor continues, check the trap itself, make sure it doesn't have a hole or something causing the water to drain out, make sure there is a trap on the line. I once helped a lady out in a house that is about your age, someone connected a bathtub up to the septic without bothering to even put a trap on the line.

Check the any basement drains, I once read the master plumber here, speedball1 say that if the water is evaporating on the regular basis, you can pour motor oil in the drain so it won't evaporate.

Less likley culprits will be things you may have to call a plumber for. Why just a week ago, I replaced my drain stack (main sewer line in the basement) from the toilet to the outside sewer connection, When I busted out the old cast iron, I discovered a hole that had been eaten through the bottom of a 2" galvalnized drain that ran along the basement floor. That was causing a lot of my odor in the basement.

Check those things out first. Also verify that your drain system is vented. (you should see a small pipe or two coming out of your roof).

PS: If your basement floor is concrete, Im pretty sure you don't have a crawl space under it, its probobly on a slab filled with dirt, if you do, its probobly something a groundhog or something dug out, and not a place where the average human would want to be. :)