To the OP, use the garden hose. You have already figured out that it is not from your irrigation system. It is always a good idea to disinfect the system each year. Put some water in your tank, add some bleach, usually less than a cup, and fill it all the way. Go inside and turn on the pump. Run each faucet until you can smell the bleach. Leave everything alone for a few hours and then drain and flush the system.
If you are hypersensitive you can get a water filter and run your house water through that as you fill the tank. If you are hyper-hyper-hyper sensitive then carry all your water with you. Go buy water in a plastic jug and tote it around. Most of this water that is sold comes directly out of a city tap somewhere but somehow it is better. I do understand that some people have "bad" water at home so this kind of system might make sense.
I have been doing this for over 25 years and have yet to have anyone in my family get sick when we camp. And, horrors of all horrors, I have even gone out in the spring without sanitizing my tanks! I also drink directly from the garden hose, rode my bicycle without a helmet and rode thousands of miles in the backseat without a seatbelt.