The original, small panel on my 2002 Suncruiser worked up until a couple of years ago, keeping my house and starting batteries charged while in storage. Originally it only charged my house batteries but I installed an Amp-L-Start between my house batteries and my starting battery. It allowed the starting battery to charge once the house batteries were charged to a set level.
Since your current panel isn't even keeping your house batteries charged, it's probably not putting out enough watts. When that happened for me, I attached a 50W panel I had to my ladder and connected it to my house batteries with a small, inexpensive controller. It works fine and keeps both my house and charging batteries charged via the Amp-L-Start. I didn't roof mount it since I have plans for 300+ watts on the roof and didn't want to do extra work. The downside is that the panel is exposed to theft and needs to be secured for travel. My wiring is also external so I have to disconnect and stow it.
You'll need a controller for anything larger than a small 15W panel like you have. Here's an article on the topic:
https://www.renogy.com/blog/solar-ch...o-choose-one-/
This is what I used for my panel. Although it's a 15W- 45W controller and my panel is 50W, its sun exposure is less than optimal and, under the best of circumstances it wouldn't put out anything close to 45W:
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
The manufacturer of the Amp-L-Start (LSL.com) says they're "no longer accepting orders". There are other options under the category "battery isolation manager". Some are two-way devices that manage charging in both directions. You should already have a unidirectional device (possibly just a relay) that allows your alternator to charge your house batteries. It could easily be changed to a bidirectional device such as a B.I.R.D.
https://intellitec.com/bi%E2%80%90di...delay-b-i-r-d/
A 25W to 50W panel should be more than adequate.
Alternatively you could replace your current 15W panel with another panel of similar wattage and avoid the need of a controller. Keep in mind that the existing wiring probably won't handle a larger panel and you'd still need to add a controller for anything larger. In order to charge both battery banks, you'll still need the bidirectional isolator.