When the dealer remounted the tank tray, did he drill mounting holes through the 2" X 5" rectangular tubes of the frame? I am not familiar with the way in which the factory attaches the tray to the frame in the TM 3023. Holes drilled through the tops or sides of the rectangular tubes may not be the best method of attachment, however, since all such holes may permit water to enter the interior of the tubes. In time this interior moisture will cause rust damage to the wall(s) of the tubes, and, might result in failure of the frame. Also holes drilled through the tops and bottoms of the 2" X 5" tubes reduce their load carrying capacity since the top carries a significant part of the compression forces and the bottom carries a significant part of the tension forces exerted on each frame member. If lag bolts or sheet metal screws were used to attach the tray, you should realize that these kinds of fasteners are not high strength ones. If you do end up using machine bolts to fasten the tray to the frame, you should purchase Grade 8 bolts. These are high-strength, alloy steel bolts that are about 90 to 100% stronger than the ordinary, Grade 2, steel bolts. Stainless steel fasteners of all kinds should be used only in locations where corrosion resistance or the stainless appearance are important. They should not be used in locations where strength is important - because stainless alloys are weaker than ordinary, mild steel alloys.
I would go along with the suggestion made earlier to weld the tray to the frame members. Welding will eliminate the necessity of putting any holes in the frame. It is very simple to do and will provide an extremely strong connection. Since homeshop MIG welding units are so common these days, one of your neighbors might be able to weld on the tray for you. If you remove the tanks and clean off the areas to be welded down to bright metal, it should take a welder no longer than 10 minutes to weld it.
One final thing you should evaluate is whether or not there is excessive movement between the two frame members. If the two longitudinal frame members are not firmly welded to each other, the movement within the frame while the trailer is bouncing on the highways will, in time, destroy any fastener. It's also a dangerous situation. :