After reading someones post on who they should see for a 518 rebuild I figured I would chime in.

A518 have a very narrow reverse band. This will be the first thing to fail on it. Trust me not having reverse SUX. Especially when you are towing a 32' trailer.

I managed to go through two reverse bands on my Suncoast tranny, both times which they replaced. The second time they abandoned the the 518 and built a hybrid 47rh/518, essencially a non lock-up 47rh. Second generation inards are a lot thougher and the reverse band is nearly twice as wide.

If you have the money have the trannsimision shop you go too build a Lock-up 2nd gen tranny. If you cannot afford that, at the very least get a Hybrid tranny.

 

The reverse band on a 518 is a single wrap design, just like the old 727. The newer 47RH/E's use a wider double wrap design. Now if we look at the rear servo piston bores, the 518 bore diameter is larger than the 47's. So for a given amount of pressure the 518 will exert more force from its rear servo piston because it needs more force to apply and hold the narrower single wrap band. I don't know what was breaking in yours, but higher valve body pressure probably weren't helping.

As far as inards being alot tougher, I don't know, the real improvement was when they put the steel front planet in the RE's. The internal evolution went something like this: (I left out 48's because I have no personal experience with them yet)
A518, Aluminum 4 pinion front planet carrier, 4 pinion rear planet also aluminum, single wrap low/reverse band.

47RH, Aluminum 5 pinion front planet carrier, same 4 pinion rear planet, double wrap low/reverse band.

47RE, Steel 5 pinion front planet carrier, everything else the same as the RH.

I'm not getting into clutch counts right now.

 

My brother has broke three low/revese bands, because he will not adjust frequentlly enough. I see others that seldom need adjsuting. If line pressure and TV linkage/cable adjsutment is off, it can advance wear rate.
Backing that 32 foot trailer is a chore with only 2.2 reverse ratio. 4WD in low would certainlly help and give better control.