Hello Thane108.
It does not sound to me like you are experiencing multipath. Multipath is interference cause by a secondary signal that is arriving to your antenna after being reflected from something like a building. It used to be a common problem with analog TV but not so much with digital TV.
Sounds more like you are experiencing lack of signal due to the trees. When the leaves grow, the signal is diminished.
What RCA antenna are you using? Some RCA antennas are quite good, but others are not so good.
Is 75 feet all of the coax that is being used? It is not so much if it is all that is used, but quite a bit if it is 2 x 75 feet. Try to set it up in a way that uses the least coax overall. Also consider that HDHomerun tuners don`t have the reputation of having the most sensitive ATSC tuner out there, so shorter coax lengths to them would surely benefit. Also RG6 Quad Shield with quality Snap And Seal connectors are a plus.
Finally, if all fails, you may want to consider using 2 antennas with separate coax runs. This will eliminate the splitter which introduces a 3-4 dB loss. 3 dB is half of the signal lost right from the start.
Michel