My old Massey had the center mount filler, a PITA to fill with the sunken small fuel filler opening. If I was still using the Massey one of the siphon ideas would be useful since a 5 gallon jug can be set on the flat hood and would be higher than than the tank.
The TN has a rear filler with a very large opening-- maybe 10-15cm-- can set a 5 gallon jug's lip on the edge and pour in, easy access from the rear. The sound changes when the upper part of the tank has about 20% empty left in it, although I look in every now and then anyway. (20% is considerably less than 5 gallons! The whole molded tank is about 28 gallons)
I would not like the idea of 300 gallons of diesel up high in a building as one person suggested, the tank could rupture in a fire just when the fire dept is trying to put the fire out (wouldn't be the first time such a thing has happened either). It would be better if the tank was visible outside so they know what they are dealing with-- and as an added bonus you don't waste relatively expensive indoor space for a tank.