A pilot reported that after a night landing at DWH he taxied on the east side of the airport with no taxiway lighting and while avoiding vehicles parked on a taxiway the aircraft departed the taxiway and became stuck in mud.

Narrative

I was unfamiliar with [DWH] as I had only landed on the field once before and taxied during the day time to the local FBO adjacent to the runway. On this particular day I arrived at night to visit a hangar on the east end of the field which I had never visited before so I was unfamiliar with the surrounding hazards at this hangar's location. After arriving I noticed the lighting on the airport was minimal and I proceeded to taxi with all available lights on my aircraft to help mitigate the effects of the darkness. I taxied toward my final destination on the field and managed to successfully avoid the unlit sea lane and other unmarked hazards. As I went to make my final turn onto Eagle Lane where the hangar was located I noticed that there were vehicles impeding the taxiway area and was unable to stay on the yellow center line of the taxiway. I looked off the right side of my aircraft and did not see anything that looked like an obstacle and neither did my passenger. I was unaware that there was grass between my present location and the ramp adjacent to me. There is no lighting on the taxiways or ramp area on the east end of the field and no caution markings for pilots who are unfamiliar with the area to differentiate between black asphalt and grassy areas which look black and blend in with the taxiway due to inadequate lighting. As a result of my forced deviation from the taxiway centerline due to vehicles and equipment impeding the taxi area I inadvertently and unknowingly went into the grass. In order to improve the safety and prevent the same situation from recurring in the future there needs to be better communication and security between airport operations and tenants to remove obstructions from ramp and taxiway areas. There also needs to be a significant improvement in ramp lighting and taxiway edge lighting to advise all pilots of the hazards.