I have a small IPTV network but working good. There is one customer that owns hotel that has some 100 rooms. It is equipped with rather good TV sets but only coax cabling. They want to provide my service but I need to convert multi-casts to RF on the spot. Obvious is to take some STBs (e.g.Amino 110) that have RF outs and mux the signals.

The question is: is there any more 'professional' solution to this? Some multiple multi-cast receivers combining into one RF or similar?

What you are looking for is to change digital video and audio to analog which would degrade both your video and your audio but if you have a specific purpose for doing it, I would try contacting Cisco which makes network, modems, gateways and other commercial and non commercial electronic equipment.  If they don't have it, they would know who does but you might try searching for "Digital to Analog" as you should bet a whole lot of indications on where to look or what to buy but look hard at the reviews on whatever you find before buying it.