FOR many years this newspaper has chronicled the challenges facing our national parks. Already underfunded, sights from Grand Teton National Park in Wyoming to Henry Wadsworth Longfellow's home in Cambridge, Mass., are facing more belt-tightening as efforts to balance the federal budget continue. Now come press reports of a $4 billion backlog in employee-housing maintenance and septic-system repairs across the park system. At the Grand Canyon, employees live in dilapidated wooden cabins or 30-year-old trailers, or bring their own RVs. At Yosemite in California, some workers live in canvas-roofed ''tent cabins.'' At North Cascades National Park in Washington State, employees get water in buckets for brushing their teeth and bathing. The park service is exploring leasing land to developers to build and manage employee housing. A bill before a Senate committee would let parks keep a bigger share of entrance fees and royalties - now about 2 percent - from park concessionaires. Congress should pass it soon.