Grey tank placement

Hi all!  I’m planning on placing my grey tank under the camper and between the trailer frame.  Problem is, the space between the frame is too small to fit the tanks on vintage trailer supply.  I think maybe there is a little under 4 feet square.  So, either I find a smaller tank to fit (suggestions of where are appreciated), or I place the tank beneath the trailer frame, closer to the ground...VTS sells a 4” thick tank.   Does anyone see a problem with the tank being that low?  Maybe a foot or less off the ground.  

Thank you