Pauleta 150 /Łukasz&Agata clearly what you are asking for is rental agencies which are easily located if you do a bit of internet research .
for which I attach ,for a starting point , the following two web search adresses . In your searches do make sure to specify the California locations you want .
https://ca.search.yahoo.com/search?f...ls+Los+Angeles
https://ca.search.yahoo.com/search?
A general car rental would suffice if you specify getting an SUV or a van which has a large floor area for rolling out a pair of pads and sleeping bags .
A rental carshould be easily arranged with your home base travel agent who sells you the airline tickets in Europe . A car or SUV picked up at the airport it will be super easy .
Some of the motorhome rentals will have small units that are suitable , but may not be available directly at an airport arrival, or require extra payment for that option . Do your research .
Pricing will vary considerably .You must factor in in the additional costs which may appear above the advertised price , such as insurance , sales taxes , extra fee if returned at place other than the start dealership .
Also consider the added cost of staying at the occasional campground for use of their showers etc .
State parks and national forest land may have low cost or free camping permitted . WalMart parking lots used to allow free overnight parking for motorhomes . Just take care in what type of area they are in ( crime )
If the cost of an SUV or mini-motorhome is too much you can also consider
renting one of the smallest, most economical compact cars , then bring your own tent and sleeping bags . You can reduce or eliminate over size and overweight baggage costs on the airplane to America by going to LA and there at a big low cost big super-department store you can buy an inexpensive camping set which you use for the duration of the trip and give away to a charity store before going home .
Shop around , Have fun on your travels in California