Based on customer provided pictures I have seen of the R23 under construction and on owner comments that there is no gauge provided to indicate the level of fuel in the diesel tank, I have a suggestion to make.
First, it appears the deck fitting for the tank is located slightly aft of the end of the tank and that the filler tub is installed between this deck fitting and the elbow fitting on the top edge of the tank. If the fuel fill fitting on the tank were changed from an elbow to a straight vertical fitting and located closer to the aft end of the tank and if the deck fitting were located slightly forward of its current position and directly over the tank fill fitting, the fuel level in the tank could be checked with an appropriately calibrated dipstick that could be inserted through the deck fitting and fill tube down to the bottom of the tank. If RT were to do this and were to provide the dipstick, owners of the R23 with diesel heat could easily check the level of diesel in the tank.
I owned a sailboat years ago with an arrangement like this. The fuel tank was located directly under the cockpit and the deck fitting was located directly over the tank in the cockpit floor. The manufacturer provided a plastic dipstick about the size of a 1/4" to 3/8" thick dowel that was calibrated in 1/4 tank increments for checking the fuel level in the tank.
First, it appears the deck fitting for the tank is located slightly aft of the end of the tank and that the filler tub is installed between this deck fitting and the elbow fitting on the top edge of the tank. If the fuel fill fitting on the tank were changed from an elbow to a straight vertical fitting and located closer to the aft end of the tank and if the deck fitting were located slightly forward of its current position and directly over the tank fill fitting, the fuel level in the tank could be checked with an appropriately calibrated dipstick that could be inserted through the deck fitting and fill tube down to the bottom of the tank. If RT were to do this and were to provide the dipstick, owners of the R23 with diesel heat could easily check the level of diesel in the tank.
I owned a sailboat years ago with an arrangement like this. The fuel tank was located directly under the cockpit and the deck fitting was located directly over the tank in the cockpit floor. The manufacturer provided a plastic dipstick about the size of a 1/4" to 3/8" thick dowel that was calibrated in 1/4 tank increments for checking the fuel level in the tank.