That shoudl work fine. You can also get an adapter and tire valve style fitting. Drill a hole and mount that to the rear bumper somewhere. I work on ambulances and they often have a manual fill valve mounted like that. Here is some on Amazon...


These are for 1/4" line but this is what I'm talking about. You would still leave and use all of the automated parts of the system, you just add these to have a manual method as well.