The biggest problem I see, is the balance and towability of your trailer. The frame and tongue are designed for the current maximum load and load distribution. Adding another 40 gallon tank forward of your existing tank is going to seriously affect the balance and total load of your trailer. 40 gallons of water is going to be roughly 350 lbs of water...add the weight of the tank, mounting hardware, bracing and plumbing and you are probably talking about 500 lbs of unengineered/planned weight being added to the forward balance point and tongue weight.
Easily done, but, will do odd things to the towability, void your chasis and axle warranties and may adversely affect the towability of your unit. If you go that route, I'd talk to several frame builders to see what affects it may have on your existing frame.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)