I’ve washed out hundreds of frac tanks back when I was in the oilfield. No idea what exactly the tanks you’re looking at but probably pretty similar. Sounds like you’ve got a good handle on it so far.
Profitable? Absolutely. Crappy, grungy work with a very realistic threat of danger/death with the stuff that gets stored in those frac tanks. Always, always, always have a sniffer and wear it below your neck line. Most of the time we were worried about H2S which is a heavy gas that collects in the lower areas of confined spaces. Spare no expense on sniffers. It may very well save your life.
First things first. Use all the PPE. There’s no such thing as going overboard and it sounds like you already know that.
Second most important thing to remember is that when doing work like this YOU OWN THE JOB SITE. I don’t care if Jesus himself walks up and wants to have a two minute conversation with you where he’s giving tomorrow’s lottery numbers. Your attention stays on the job 100% of the time. You don’t let people walk through your job site. You don’t let people stop and talk. Nothing. One minute of inattention could cost someone dearly.
Seriously, when you start working LOCK IT DOWN.
Anywho 
For us, we didn’t get a lot of calls for tank cleaning specifically though there were some. It was typically one of those “well, since you’re here” type things and all of our crews were CS trained. I believe we charged around $125/hour per machine and then an additional $25/hour per man with 4 man crews. Keep in mind, those prices were nearly a decade ago. Work in rotations. Two guys on the trigger with one man resting and one man on “guard duty” aka he’s there in case people start dropping. I always ran 20 minute rotations when doing CS work. With a good crew you can definitely stretch that out a bit but people have a tendency to get edgy when they’re in tight spaces for too long and that limits their productivity.
Vac trucks are necessary. No getting around it, really. The frac tank drains are typically off the bottom by roughly a foot so you’re not squeegeeing the muck to the drain and even if you could you can’t let it hit the ground.
You can rent a vac truck like you mentioned but I would urge you to familiarize yourself with how to operate the vac truck yourself as well. Simple remote systems and it is ideal to have a guy in the tank that can run the controls as they do the work.
You’ll go deaf if you run that vac truck wide open the whole time you’re in there. With a guy inside running the remote he can shut it down and fire it up when desired.
Do as much work as physically possible from outside the tank. Use long extensions. Suck it down to below boot level, get in there and turn it into a slurry with turbo nozzles, and shove the suction pipe in a corner because they will jump around as the suction comes on and goes off. Put your foot on the pipe before turning on to help keep it from whipping around and hurting someone. Again, incredibly loud. Wear ear plugs. Also, ear plugs help keep the brain eating amoebas away.
Keep plenty of squeegees and replacement handles on the trailer. They break. All the time. I know what you’re thinking–just swap the wood handle out for metal. A) it’s heavier and B) too dang slick. Wood is best but just have to replace handles often.
Radios are pretty critical for this type of work. If you ever lose or break a vac truck remote it will essentially shut you down if you don’t have radios. You can operate the truck from the control panel onboard but with the noise and being parked away from the work area, you’re SOL without walkie talkies.
Magnetic mechanics work lights. Buy 'em. Inside metal tanks you can slap them on the walls or ceiling and actually see what you’re doing. Wish those were around back in my day. We just had the “guard” holding a mag lite. Not great, Bob.
There is actually quite a market for stuff like this that 99% of washers do not do. Hydro excavation, car wash pit clean outs, grease traps, etc. I’ve considered offering hydro excavation but I have no interest in doing any more CS or really any industrial work again.
Hope that helps somehow!
