Andy,if it seems like a good ideal to you go for it.
If it was me I'd pick a spot out in the far back corner with room to work around the truck.Don't know what kind of deal to tell you to work out with the truck stop for using the spot.May be able to work a deal with them on keeping their flatwork clean at a lower cost for the use of the spot.
Your going to run into alot of different trucks Like Dan said,I figure most will be independents trucks,most company driver will not get theirs washed there.You'll see alot of fancy rigs with chrome,polished,ect.,,if you get to wash there,you'll learn how to clean the fancy rigs which will get you alot of bussiness later on.These guys are picky so be careful on what you use to clean them,it doesn't take much to do damage to polished rigs if using the wrong kind of soap or acid.
Another thing,are you going to set there alday every day of the week,,than again you may not be setting but working.I haven't seen nobody stay at a truck to long like that b/c they couldn't make it work.....
If your wanting to wash independents,take a drive around you on weekends looking for them,there are more than you think in your area.You'll see them setting here and there and most will have a company name on them as to who they are delievering for.Most will know others in your area,just ask if they know of anybody else thats independent.