Better monitoring of freshwater level

I have an alternative method installed. It's called my eyeballs.   

Seriously though, the simplest solutions are usually best. You can see the water level in the fwt. If you want to be able to check it more accurately why not fill it, then drain say 5 gallons at a time and mark the water level with a marking pen? Assuring the tank is level shouldn't be a problem because you're levelling the trailer anyway.

If you don't want to have to bend down to check every time just reach down and snap a photo of the area where your marks are with your phone.