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? 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.
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.