If you are going through Lake Havasu City there are 3 BLM camp-sites, one just south of the city on the east side but the exit off the highway is pretty steep through a ditch. We went to the one north of Lake Havasu City about 5 miles north on I 95 on the east side, a wide open area with nice views but a bit windy and you can stay as long as you want. The one that we should have stayed at was just north of the Havasu airport again on the east side, can't see the sign from the highway but it is the first turn after the airport on the north side, very nice, it's in a canyon with lots of sights. Best part is they are free and lots of nice people there! Again stay as long as you want.
Hope this helps, better than staying in a service station.