I was kicking around ideas this weekend to reduce my tongue weight.
Yes, you could cut down the support and put 10 pound tanks on, but I was just looking at options in the 2013 rv parts catalog from dealer. They make composite 10, 20 and 30 lb tanks.
The 20 is listed as 50% lighter than traditional steel tanks.
To be up front, the brand my RV dealer sold, "The Lite Cylinder Co.", have all been recalled. I think the company is effectively shut down.
Another mfg, Ragasco, makes tanks primarily for the forklift market.
Here's a link to their site.
After a quick search, I found an RV shop that sells them.
I can't speak for this shop, but you can probably call Ragasco direct to find a retailer.
Whatever you do, don't buy the Lite company brand. Some sites appear to still be selling them.