I sometimes get flamed for saying "There are now six volt batteries in RVs"
And yes, I know the do put six volt batteries in RV's. I mean I have two of 'em myself, but when you istall them something magic (not really) happens.
The two six volt batteries, wired in series, become, a single 12 volt battery.
So you said you are putting in 4 Trojan T-105s.. I would say you are putting in 2 Pairs of Trojan T-105s..
When you see the difference, you will have your answer
If you can not see it:
Each pair, in series, is a 12 volt battery
So you are asking: Can I parallel a 12 volt battery with a 12 volt battery?
And clearly the answer is yes.
The "Dangers" are mostly myth but there is one that's real.. IF the cells in the older battery fail SHORTED,, it can cause the bigger and newer Trojan's to dump charge (current) into them far faster than they can accept, this can lead to the older battery "Exploadig" (Low level explosion but it sprays acid all over so it's a pain to clean up)