2007 Slow starting/fuel guage drops

I have a 2007 crew cab, 2-wheel drive. I do not have any modifications to the truck. A few months ago I had the Toyota battery replaced with another due the battery discharging.

This morning (after sitting over weekend) I tried to start my truck to find out it cranked very slowly. The dash lights went out or flashed. After the second crank my truck did start and i was able to make it to work.

I just went out to try my truck again, the truck starts but when it does start, the fuel guage goes from 3/4 full to empty then back up again. Also, the battery has 14v when running but slowly discharges when turned off. The only modification I did within the last few weeks was change the spark plugs and air filter.

Is there something else I should be looking for? The connections on the battery are tight and clean. The connections on the starter are tight.

Do you think the new battery is bad again? Should I not be getting a Toyota battery? This is the second 84 month battery in a few months for the same cause...I think.