First, understanfd the Tamiya color you chose is not the same color as the US Army and Marines use. If you need to use acrylics, try to find Polly Scale. They have an exact match to the current desert tan.
Okay, before you weather, lay down a gloss coat of your choice and apply the decals. Seal them with another layer of gloss when they're dry. Now, do what Armorguy told you. Seal this with flat coat.
I use Testor's laquer gloss and dull coat from an airbrush. If you don't have an airbrush, you can use their aerosol cans, but be sure to have a lot of ventilation and shake the cans very vigorously, especuially the dull coat.
You can also go to an arts and craft store and pick up some chalk pastels. Chalk, not oils. Scrape the flat side with a knife edge to make a fine powder. Various browns to match the terrain and black for exhaust. Apply this to the lower section of the model with an old brush to simulate dust. If there's an exhaust port/pipe, run some black in the direction of the exhaust to simulate staining.