Cessna 150 pilot reported loss of engine RPM during cruise. The pilot diverted to make a precautionary landing.

Narrative

Was CAVOK. Unlimited visibility. Was in cruise at 7;500 feet and engine began to lose RPM. I tried manipulating mixture lever; throttle; checked fuel selector; carb heat. Nothing helped. I wasn't able to maintain altitude on reduced power; nor was I able to 'fix' the issue; so I [requested priority handling] with ATC (I was on VFR flight following); and with not many open fields around; nor knowing how the engine would perform as time passed; I opted to follow the road until I could assure gliding distance to ZZZ. Engine performed better at lower altitudes and momentarily with selection of carb heat. Landed at ZZZ with engine power.I have not flown this 150 much so I called another pilot who flies it often. He said this plane doesn't like the higher altitudes ('that's airline pilot height' :o) and perhaps the mixture creeped higher because it's not the type with a lock. Who knows? Oil pressure and temperature were good; and visual engine inspection didn't reveal any issues. Oil quantity was within range. We had three hours of fuel onboard.Since the engine performed well at lower altitudes and a subsequent engine run up (done on the ground) showed no negative indications; I decided the loss of RPM must have been caused by the mixture being set (or creeping up) to too rich in cruise. I took a 30-minute decompression break and decided to continue to our destination at a lower (non-airline pilot) altitude.I would have liked for this engine to not have been temperamental; but given the circumstances; I believe I chose the safest course of action. Perhaps the one thing I could have done was check the mixture lean a second time; or multiple times; after setting it initially in cruise. Or chosen a lower altitude. I am thankful for ATC giving me priority; just in case it was a more serious issue; and for the Fire and rescue personnel waiting on the ground.