Mixing batteries is very bad in every combination. The weakest battery in the string spends it's entire time drawing from the strongest one(s).
As Dick said, the charger doesn't like it either because it will respond to the demand from the weakest battery and overcharge the stronger ones..
I am a fan of the Trojan T105. I have had several sets over the years in two coaches, with the expected performance. Also like Dick said, I check my battery water monthly. Get the Profill water setup. It isn't cheap but it makes watering a string of batteries a piece of cake! I spend less than 5 minutes doing my 4 Trojans.
2003 Tradewinds LTC, Dynomax Chassis,
350 Cummins, 2008 Lexus RX400h, 18' PJ Trailer