1) If you just want to fill the tank with a garden hose, 5 gallon jug, etc, a simple gravity fill will do.
2) Shurflo sells a lot of RV pumps. They've been around for a while, get generally good feedback, and you can actually have a useful conversation with their customer service people. Get one with a higher max flow rate (GPM) than you think. The worst that happens is you get better water pressure. Just don't overpressurize your plumbing with some crazy 150 psi RV pump.
3) Not totally clear on what you want for functionality. Do you want the option to be able to use pressurized RV-type water hookups? If so, definitely think through the check valves. Most auto on-off pumps (common for RVs) have a check valve built in at the outlet. Make sure your pump can self-prime if your water supply is below the pump. Otherwise you can run it dry and/or have cavitation...either way, the pump will destroy itself shortly.
4) Haven't found a good one yet. If you find one let me know!