It does appear to be a simulcast system. These can be a bear to monitor with a scanner.
You are probably experiencing multipath simulcast distortion. Depending upon where you are, your scanner is receiving two (or more) transmitters at the same time. For a variety of technical reasons, the signals may be out of sync with each other and your scanner is having trouble decoding them. (Sort of like two people talking at you at the same time and you have trouble making out what both are saying).
This usually manifests itself in either garbled audio or no audio even though the system is transmitting. As you noticed, when you get closer to a site, the problem diminishes. That's because the scanner is getting one good clean signal.
As odd as it may seem, the key is trying to get your scanner to receive more poorly. That's so it is only attempting to receive just one of the sites.
So the first thing to try, as noted above, is to program the sites so that the attenuator is on.
Additional things to try are changing the P25 Adjust Level and P25 Adjust Mode. These are "hidden" settings. To access these, to turn the scanner off. Then press and hold, the Hold button while powering the scanner back up. You'll see the display cycle through the colors.
Then go to the Menu>Settings>P25 Adjust Mode and Menu>Settings>P25 Adjust Level. Change the values of these.
Unfortunately this is a "trial and error" process. You need to come up with a combination that sounds the best to you. I can set my scanner at 13/175 and it decodes reasonably well. My son has a comparable scanner who uses something like 11/178.
Something you may suggest up the chain of command of your fire department is something our fire department did. We're in a similar situation - we're on a simulcast system and not all members can afford scanners. What we did was set up a Broadcastify feed. We've taken a system radio (which doesn't have the simulcast distortion issue because they use "pricier innards") and feed it into a computer for radio feed use.
Then those with smart phones can monitor the feed and stay informed about an incident. Unfortunately there is about a 30 second delay between transmissions and the time they get to the smart phone.