I'm installing a 30 gallon fresh water tank in a DIY weekender and it comes with 3/8" fittings. Should I install 1/2" adaptors and run 1/2" pipe or will 3/8" be adequate ? I'm also looking at Shureflow water pumps and was wondering if their Aqua King 3.0 GPM would be a better choice over their regular 2.8 GPM, concidering they are practically the same price.

The pressure side is between the pump and faucets. A 3/8" line has the capacity to flow ~3 gal/min at 40 psi, the out put of your pump.

If you installed adapters and 1/2" lines, the water will still flow at the 3/8" rate, IE, the fitting on your tank. The only time a larger line would be beneficial is if you had a long run, when you could experience some line losses. You shouldn't experience this in a 4' weekender. The higher the pump's volume, the more useful it is. However, I doubt that any differences in the actual flow rates between the two pumps, at the faucet, would be noticeable. Go for the best construction and reliability.

Gard

The bursting pressures of PEX are typically tested  higher than most households and your pump. This type product is typically used in hot and cold feeds, and has proven to be very reliable. Both of my LQs are conversion company equipped with PEX, and we recently installed it in one we built.

It's a great product that's here to stay for a while.

Gard