See how you can make out with what you have. Take notes about your energy budget, i.e. are you just using nav lights, or a microwave and entertainment center?
All we're using atm are the instruments (Raymarine depth sounder and knotmeter, and Garmin 498 GPS), VHF radio, and the engine starter. We used a few of the cabin lights briefly
the night we over-nighted in a marina, bringing her up. We have no microwave on-board. Nor any entertainment center, other than... hmmm... I haven't even looked at it closely--I think it may be just a CD player. I dunno, maybe it's a radio, too. (Can you tell how much we cared about that accessory?) We probably won't do any night-time sailing, at least not intentionally, this season.
Some things can be made more efficient regardless of how much power you will have, and "for now" if you have to run the engine for an hour daily to keep the batteries up--just do it, and make good use of the motoring while doing so.
We have nearly a 1-hour motor to get to/from the area in which, at our current level of expertise, we feel comfortable raising/lowering the sails.
I think our energy budget will have a surplus for the immediate future
A lot of surplus houses i.e. www.allelectronics.com
have simple digital voltmeters for sale under $20, these are raw displays with electronics behind them that can measure 1-20V with good accuracy, so you can install one with a push-button switch (to turn it on when you want to read it) and check the battery state without spending a lot of money on anything fancy for now.
Thanks for the pointer. Maybe I'll go that way.
I wonder if you'd even need the push-button? Maybe just a "1/2" switch? A voltmeter should have exceedingly high resistance. I would imagine a lead-acid battery's self-discharge rate would exceed any drain from a voltmeter. In fact: Why even a separate 1/2 switch? Just pick it off something on the "output" side of the existing off/1/both/2 switch. The ol' KISS principle. One less thing to break. One less bit of additional wiring.