chassis battery charge ?

Your description of how/why you think the start battery isn’t charging is a bit suspect. Because if it starts it starts. And has not sure how battery charge is related to fuel. IE it isn’t.
But is that why you drive it “a few miles every 10 days”? To charge the battery? Also an odd routine. And a few miles won’t do much ifnthe battery is already low.
Why not just throw a battery maintainer on it and not worry about it? That’s the cheapest easiest solution and requires no time nor effort for your periodic short drives you’re doing now.

When I owned a motorhome, I installed the Trik-L-Start like was mentioned above. It worked great. Highly recommended.

don't take my word for it...

https://www.consumerreports.org/cars/car-maintenance/how-to-maintain-a-car-that-isnt-driven-much-a6412749580/

https://www.firestonecompleteautocare.com/blog/driving/how-short-distances-affect-your-car/#

https://www.tiresplus.com/blog/driving/driving-short-distances/#

RK, explain the problem with short jaunts and engine not getting to proper o perating temperature. What happens- what are the consequences? Is that just a battery issue or something else? Doug

Hi,

This is a good unit.

https://www.bestconverter.com/TRIK-L-START_c_210.html

There is a higher capacity one called the Ampl-L-Start.

no way to know. likely would depend on the level of discharge. But, of more concern would be running the engine for short periods before engine gets to proper operating temperature. A 30-minute drive once a month should do it. Recommemd you reconsider the Trik-L-Start device. Inexpensive ans EZ to install.

Trik-L-Start

RK thanks, I will pass on such a device for now, but wonder if I take the rv out every 9-10 days how far would I need to drive it to keep a chassis battery charge up? Doug

In the motorhomes we've owned plugging into shore power charged only the house batteries. But, we added a device...can't recall the name...that diverted a small amount of energy directed to the coach batteries to the chassis battery keeping it charged.