With the limited information provided, let me take a shot at your concern. You seem concerned the RSSI varies 10 to 15 dB as you listen, at a 7 mile distance from the repeater. Being you are in New York state, I would first assume there are many trees in the 7 miles between you and the repeater. Signals in the 450 band are affected by foliage, and particularly damp foliage. There may also be some building or hills in between.
Lets convert everything into dBm to compare. Let's say you mobile and repeater operates at 40 watts [+46 dBm] and both receivers at 0.5 uV [-113 dbm] for 20 dB quieting. This means your "link budget" is 159 dB for satisfactory communications. If you subtract the path loss of 107 dB for a line-of-sight path, then you would still have a 52 dB fade margin. [sorry for the microwave link budget approach]
Another way to look at this is taking the 40 watt [+43 dBm] mobile and subtracting line-of-sight path loss of 107 dB, will yield a maximum signal at the repeater of -64 dBm. If your -80 dBm repeater receive signal is correct, you are losing 16 dB somewhere along the way. This could be trees, buildings, hills, etc. If the signal varies while you are setting still, then it is likely you are not receiving the signal line-of-sight and therefore the signal will vary due to reflections, refractions, polarization changes, and phase cancellation.
This is why cellular base stations use dual receivers with dual antennas, that are often times different polarization. This yields an improvement of at least 15 to 20 dB.
Anyway, get use to the signal varying when you are not line-of-sight, and sometime when you are.
Last, to determine if your other antennas are affecting your numbers, simply go to a large open parking lot and drive in a circle, with and without the other antennas, to see the difference.