Not being an expert and having only second hand information from experienced people I really don't understand your concern with your septic tank as long as none of the baffles or the drainfield is damage by driving over it. One of the professionals that came to the nursing home I worked at to inspect a very old septic tank - over 50 years and you could probably add another 25 years to that (it had once been a loggers bunk house) said it would continue to work unless the drainfield got damaged - all the water going in it just goes out the drainfield and the tank its self needs to be full in order to work and drain. The amount of tank water you put in won't hurt a thing because it contains nothing that would damage the chemical action going on in the bottom and has no solids - it would just run off. A working drain field is very important and so many people don't consider that when they pour concrete slabs to built garages or other sheds. When you get into country like where I live a lot of septic tanks got put in before you needed permits - or if you did need them, they moonlighted them in. Then too, before the age of computers those things weren't recorded very accurately either and even if they did, some of them never got put on the computer records. This I know from experience. We wanted to built a hangar about a thousand feet from our house and put a septic tank by it for our RV. Couldn't do it without first proving where our house septic and drain field were located and the county had no record because it had been put in 40 years ago. We knew the original owner had a permit but no longer knew exactly how the drainfield was laid out so I had to guess and draw a map and we finally got the permit, but it had to be an engineered system - for a septic that is used about once a year.