Thanks for all the helpful replies.
I decided to do exactly what Bill suggested, mechanical removal via cutting access holes and welding back the holes again.
I removed the insulation, and by tapping the full tank it is obvious where the 3 baffles are separating the 4 compartments. A professional marine steel worker will cut the 4 access holes and weld them closed again at the end. All the fuel will be removed by another company and cleaned (if possible) and this other company will clean the tank too. (I would like to get my hands dirty, but I simply won’t be around along enough for the job to be finished.
I mentioned the danger, cutting with diesel fumes, but I will mention it again. He said he woud not do it if the engine was there, but I think that was mainly for the physical access to the tank. (The volvo is in a workshop.)
4 Vetus insulation panels (60 x 100 cm) will cover the tank.
I will Also take the opportunity to install a second feed from the bottom of the tank and a second return tube at the other end in order to install a fuel polishing system.
Travel Bug SM 151