Yes, I have 2 T105s, 2 L16p, and 2 a bit smaller than l16p but much larger than 105s. That's 6 6V batteries. I only use them in sets of 2 matching though. Probably close around an 800-1000amp total at 12V.
I have not been using the truck and batteries nearly as often in the last few months or so as I had been. I did take them out for awhile. I checked mileage with the battery weight and after taking them out. There was not enough difference in milage to even see it on that truck. I mean driving a bit faster or slower, sitting at an extra traffic light etc... I think had more effect on mileage than the weight of the batteries did.
I put them back in again now.
It was not much work since I set the truck up for them. Dual battery isolator under the hood so I can charge the truck battery and deepcycles same time, never drain the trucks battery. I installed a toggle switch and amp gauge plus digital meter. When I want to charge them while driving I flip on the toggle switch, don't want to charge then flip off switch. Digital meter shows me the Volts so I don't over charge. The amp gauge has a bit of problem and does not read correct, old one I had laying around, but still gives me an idea if I am charging at 25amps or 10amps anyway.
The cables from the truck to charge the batteries has booster cable clamps, just pull them off one set of batteries and stick on the other set when I want to switch between the ones I want charged. So only trouble is deciding which set I want to use and connecting the load to it.
I will be doing the same basic setup in the 1966 Chevy motorhome I bought. It will be a 24V system as planned with it's own gennie on the engine. 1750watt sinewave UPS for the inverter. I been playing with the UPS in the house now for quite awhile. Had a couple problems come up I have to solve before I get really into rebuilding the motorhome.
11-12mpg, can't hurt that much 
Though I do have plans to improve it alot 
