Incorrect (though, you could look at it exactly the other way around--put $18K toward traditional with CZTE money, then put $18K toward Roth). The $18K limit refers to the "elective deferral" limit, which applies across all similar employer-sponsored retirement account--TSP, 401(k), 403(b), etc., whether traditional, Roth, or a combination. If you're AD, the 401(k) issue doesn't apply; if you're Guard or Reserve, it's important to understand that all the limits involved apply to total contributions to ALL such accounts. Roth contributions count against the elective deferral limit, regardless of whether they're "after tax" because they came from tax-exempt earnings or if they've actually come from taxable income.
If you want no-kidding source documents, you'll need to dig through the IRS' instructions; otherwise, the TSP site makes it pretty clear: https://www.tsp.gov/planparticipation/eligibility/contributionLimits.shtml
If it's important to you to max your Roth TSP, AND you want to maximize your total contribution above the elective deferral limit (via CZTE money), you must avoid putting any CZTE money into the Roth. While deployed, put as much as you can into your traditional TSP (I'm assuming you're not above the officer limit, i.e., you get 100% tax exempt), being careful not to exceed $35K ($53K - $18K). Then, during non-CZTE months, put your money into the Roth side. Adjust as required based on your specific situation (number of months deployed, how much you can afford to put in, how close you can get to the limits, etc.).
Short answer: no.
Longer answer: CZTE money is month-by-month--you can't "save" it up to be used in another month. TSP contributions have to be made directly from earnings (you can't make a "deposit"). BL, you can only put money "above" the $18K limit *to your traditional TSP* during CZTE months. If you want to max out your Roth, "save" at least $18K (don't contribute more than $35K of CZTE) for non-CZTE months, then make those contributions when you're home.
See my reply to Disregard for more details.