Very little of the heat radiated from the nozzles would be directed toward the shorter oxidizer tank, but I'd guess maybe 10% of the radiant heat would be intercepted by the fuel tank.
The NASA paper "Characterizing GEO Titan IIIC Transtage Fragmentations using Ground-based and Telescopic Measurements" discusses efforts to analyze the materials used on the Transtage in order to characterize space debris from the stage's occasional breakups in orbit.
It has a little bit of information on the tanks:
The asymmetric small-diameter fuel and large-diameter oxidizer tanks were exposed to the engine plume and space environment and used painted and foil insulation for thermal control [during?] the long-duration missions flown.
A diagram in the paper shows both the fuel and oxidizer tanks using "aluminum silicone paint beneath steel net" and "gold plate SS foil beneath steel net".
The analysis was done on a test article, some components of which were not faithful to the stage as flown, but:
...general propulsion module structures, select electronics, the aft close-out/heat shield panel, the main engines and engine functional equipment, including engine bells, and the fuel tank are considered authentic original equipment. For example, the engines display considerable wear from firing, and the fuel tank is correct and displays the accurate types of insulation for this vehicle.
It's not clear to me from the article whether the "gold plate SS foil" and "aluminum silicone paint" were used on the same parts of the fuel tank, or on separate areas.
At any rate, the tanks had some sort of insulating coating, and the sheer mass of propellant in the tanks would also absorb a great deal of heat.