i put 3 drains (center) in my 1400 square foot garage i wash my rigs inside , winter and summer .
i did all of the following :
take the old tv from your house and put it on a shelf where you can watch formula one , indy car , NFL , inside.
up high mount large speakers so you can play jazztrax while you are working on your cars .
install a water heater . it will take the chill off in winter by osmosis. plus you need HOT water to scrub the bugs off the trailer front , scrub the tires , and clean camping gear .
although i installed flourescent lights when i built mine i have discovered that multiple switched porcelain cheapy light fixtures work better . put CFL bulbs in all of them .
you are gonna want maybe 5 or 7 light circuits switched so you can focus on your working area .
install one porcelain CFL light that is not switched but rather is on 24 / 7 . that way when you walk in day or night you always have light to find the light switches .
the obvious : 240 volt receptacle to run your welder a 30 amp 120V circuit to run the RV for the day or 2 while packing / unpacking.
plenty of 20 amp receptacles to run radiant heat dishes where you are working . a small compressor is handy .
obviously a roll away tool chest and a work bench .
we have a conventional refer that is always full of cold soda , ice for the ice chests , various frozen meats , all handy when packing to go .
steel shelving ( gorilla rack , costco ) that is portable yet will hold up heavy objects . you may move them later as you determine the best spot .