Right after I moved and hadn't had a chance to wire my 60 gal compressor up, I broke down and bought a little 7 or 10 gal pancake type compressor from the store with the big orange sign (I HATE that place, but it's the only hardware store for 30 miles. sucks to live in the country sometimes).

Anyway, I paid about $100 for it and it does the job just fine. I've run my 1/2" impact off it (just barely) as well as aired up tires and run an air nailer. it's actually pretty durn handy.
You can suarely fill your portable tank off it, just be sure to wear earplugs when you fire it up cause it'll be running for 10 minutes. I'd fill until it starts sucking wind, then stop filling and walk away for a minute or two. then come back and do it again after the compressor has had a minute to rest.

OR.. just do what I do now and toss the whole compressor in the trailer. it weighs just a few pounds more than my portable tank and holds almost as much air. fire it up at the house and let it fill up there, then truck it to the track and only plug it in if you need more air.

The benefits are many-fold since you have enough air at the track to run an impact wrench and so on so forth.