a.) Does anyone know of a solution that I can pour in which might seal the leak internally?

b.) What is involved with removing the tank for inspection/rectification?

Cheers.

As for dropping the tank, it shouldn't be hard to disconnect the plumbing fittings, the wire(s) for the level indicator and the bolts, but working from underneath to do it, well that's another matter.  It would be a lot easier done from a lift of some sort.

Shouldn't be that hard to remove, and if you can't see where the leak is then you can't fix it, so it needs to come out. M suggestion is to remove it, fill it, find the leak. empty it, dry ti out, glue the leak up with ABS cement, refill, and see what happens before reinstalling. If the leak hasn't been fixed, you haven't lost much. 

I shall try FR for more information.

Again, thanks.

ABS schedule 40 pipe is more rigid than a flat tank because its thicker and the circular shape resists flexing. Here is a link to a set of videos showing show some RV ABS tank repair techniques. If your leak is a crack with a sharp tip or tips it will continue to grow unless you stop drill it first. The drill holes relieve stress at the crack propagation points, so you have to place the hole in front of the direction of crack growth so the crack grows to the hole and stops. 

A dealership has told me that the grey tank is polyethylene, not ABS, and "nothing will stick to it". Repairs to stop leakage with this type of material has to be done by 'heat welding'. I shall be making an appointment soon to have this done. 

Meantime, I applied some Lepage PL construction adhesive to the outside of the tank and it did indeed stick,  but I do not know how long this will last. I can see no way and have not discovered how to remove the tank. Thinking outside the box, I played with the idea of extruding ABS cement (now not an option) or PL through a wand inserted into the tank and doing a trial-by-error application method, but this may well prove not only ineffectual but contaminate the area such that the heat weld option may not be viable.

Additionally, there is no way of currently knowing whether this is a puncture hole (not likely as it is about halfway up the tank, protected by a steel crossbeam), or an abrasion following travel on a rough gravel road. 

Maybe I should stop taking "Arnie the Podinator"  down remote roads!!!!

I will update everyone after the repair is done on cost, etc.

Cheers.

Thanks for that!  I don't know how the feed pipe into the tank is affixed. I assume it must be flexible so it can be detached before the tank is removed and replaced.