How much depth in the chassis rails is required to fit an underslung tank please? We have a 2003 MH with Alko chassis rails and were advised a while ago that there is insufficient depth to fit a tank.....so left it as was, but the possible need has re arisen again and I'm looking to re visit the situation. Not at home at the mo' to be able to measure up but the rails are on a 2003 Burstner Ducato and the MH is 6.9mtrs long overall with a rating of 3.8t.