Here is a great blog on solar if you dig around in it. This guy knows what he is talking about. I think you may need a better controller as that is the key to getting a good charge. Also the proper size cables between stuff. Too small of cables and you are really hurting your efficiency due to voltage drop.The 15v isn’t necessarily bad if the amp output is low. Bob in his blog talks a lot about charge profiles, most controllers are fixed but the cheap Chinese one I bought has software that can be used to custom configure it. I bought the cables etc to do that with my android tablet but haven’t done it. I have a tracer 20 amp with gauge which I really like. So far with my 160w single panel it charges my two 100 amp 12 v in parallel interstate deep cycle batteries really fast on the stock charging profile. I ran my RV furnace at the Swell every night and my batteries were topped off everyday usually by afternoon.
This is the controller I bought in 2016 but they may have an improved model. Bob recommends a really good controller and a good meter to monitor but I am doing fine with my set up. A volt meter and a hydrometer are tools you should have. My meter with the controller tells me all else I need to know.
EPEVER 20A MPPT Solar Charge Controller Tracer A 2210A + Remote Meter MT-50 Solar Charge With LCD Display for Solar Battery Charging https://www.amazon.com/dp/B01GMUPGX2..._dyaZAbTCYKG8T
80w of solar should do a good job if you don’t draw down your battery too far. What ever your bank is don’t go below 50% of charge for lead acid. This means if you have one 100 amp hour lead acid battery you should only draw out 50 amps. How do you tell that? By voltage. If your battery reads @ 12v that’s near 50% I think. 12.78v resting depending on temperature is full and 11.5 v is almost completely discharged. It will go less but you are ruining the battery below 11.5v