Chassis battery replaced still need to use Aux start

Four thoughts.. One Do you have an ammeter or voltmeter or do by "It shows it's charging" do you mean the idiot light went out? I will tell you a short story about that idiot light.

Second, Check the OTHER END of the battery cables.

Third: Bad starter drawing way too much current.

Four: Believe it or not, a bad ground on the alternator housing (Even shorter story)

I've seen all 4

The Idiot light story: The light does glows when the alternator output is NEGATIVE, that is its drawing more than it is putting out,, Once the alternator puts out at least as much as it is sucking, the light goes out... So if the fan belt is slipping or if one or more diodes are bad.. The light goes out A volt meter will find it.. I tore my hair out figuring this out on a Ford Granada, finally figured it out and fixed it, then found at least 3 or 4 more cars that had the same issue,, NOTE: These all had loose V-Belts.. A serp belt should not slip like ours were, but that still leaves the diode..

Story 2: That was my wife's car too,, I findally started poking around with a volt meter and found some difference in potential (Voltage) between the housing of the alternator and the negative battery terminal or engine block (Forget which) Pulled one of the mounting bolts, installed a STAR washer under it and no problem any more... That one drove me batty till I found it.

I can tell you ... Finding it was a great relief.

Oh, one more short story and thought.
Years ago (1977) Bought a new car, Well I think it was still 77 but it might have been 78 when the battery died (yes it was 78) still under warranty.

Went to J.C.Penny Auto and they put in a "Lifetime" battery this battery carried a lifetime warranty.. Lasted about a week and they put in a new one.. That one lasted many years and they put in one last new one (A reasonable length of time) that one outlasted the car. (Thanks in part to a drug overdose.. Yup, The Sentry Drugs shuttle truck ran right over the car, Thus it died of a drug overdose).

Only hard to start after its been running for a while? Starts fine when cold?

Do you perform any of your own maintenance? Do you own a 12 volt test light or a hand held multi-meter?

Measure the charging voltage on the actual battery terminals.
Otherwise sounds like a loose cable issue that still exists.

All connections to the battery need to be checked for corrosion, at the battery, down the cable and at points of connection to the frame for ground and starter for positive. The charging only indicates the engine is sending the charge out, but if it can't get to the battery due to corrosion, the battery does not recharge. The ground is the place to start, as when you use the assist it perhaps is getting a ground through the house batteries and then draws current from the other battery that you just replaced. Baking soda and water cleans off oxidation of connections as does a wire brush to the points of contact with the frame where the ground is picked up. Remove the nut or bolt holding the ground wire to the frame as just cleaning around it is not a good idea. Then spray the connection, (after it is tightened up), with battery terminal spray so corrosion stops.

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Start by locating your battery disconnect switches. Cycle the chassis battery disconnect to off then back on.
Your system may show it charging but it could be showing charge to the chassis battery but it is really charging the coach battery.
There could be a battery control center under the hood depending on make and model that has a silver charge relay. This could be bad or has a control fuse blown.
Post make and model of rig and what chassis your rig is on.