Gas is much cheaper in the USA than Canada, the price jumps in California. Generally speaking, in California ARCO will consistently have the lowest priced gas, fill up whenever you drive by one.
Shower and fill up water at the beach in California, it will be difficult for you to find a place to sleep nearer the coast without risking a ticket, but the beach is a great place to spend a day cheaply.
Avoid state parks, especially in California, there is usually a costly entrance fee to use the beach and carpark for the day.
Use Free Campsites for suggestions of low cost, free campgrounds in California (and elsewhere). Try REI, Walmart, rest areas that allow 8-10 hour stays, pull outs etc. Small cars are usually fine if you ask permission at the store, it’s the larger RVs that are more problematic.
Insure you car with the maximum liability coverage. The USA has a heavy suing culture, and in the disastrous event of injuring someone you will need to be fully covered and prepared to be sued.
National Park campgrounds in the States are often extremely busy and you will have to pay to stay, if you meet another group of travelers you can ask them if they would like to share a pitch for the night, saving you all precious dollars.
Fast food is a huge temptation because it is everywhere, save it for a rainy day or when you are seriously craving it after a hike.
In California you are entitled to free air when you buy gas, ask the attendant to turn it on to keep your tyres well inflated. You can also ask in other states if they will turn on the air for you after you purchase gas, usually they will oblige.
Free WiFi can usually be found at McDonalds, Walmart and Tourist information centres.