there are several (up to 5 lately) 1100RTs on ebay, mostly between 50-70k on the clock. Looks like $3,500 is about the sweet spot.
I missed out on one with 45k by $25... I'm leary of ebay site-unseen auctions but I think I missed a good deal.
I am no means a boxer expert; I only had my 1100R for less than 2.5 years and just got an RS last week. I've had literally dozens of bikes over the years though and currently have several.
I was looking for something to replace my R1100R and orginally considered the RT. I went for an RS but now want both
... and a GS one day.
Its the same engine across the R-RS-RT - I think a different final drive ratio on the RT to help with the extra weight, and i think a cam difference on the GS.
Seating position is very different from RS to RT
On the R i sat mostly upright, well balanced
RS - 'clip on' style bars, rubber damped, multi-adjustable (in/out and back/forth only- not up/down) and you are in more of a sporty stance. Less comfortable than RT, but more fun when hitting twisties. Smaller windscreen than RT and manually adjustable.
RT - 'couch' style seating. Giant windscreen compared to RS and electronically adustable. RT carries I think over a hundred pounds more weight with the same engine, so it will be less responsive than RS
i went with RS because of lighter weight, and with the bags removed its less of a full-touring bike. My touring plans for now are just overnight rides in Texas area. RT will always be in touring mode basically, with or without bags. However, even with an aftermarket screen I still get more buffeting from the wind than i had hoped.
The extra weight of the fairing can add to fork dive - but unless you've ridding other BMWs to compare, there should be a drastic difference between traditional forks - if not then as others mentioned its likely a worn shock.
the 1100's seemed to have less issues than the 1150's just from looking around.