A 40 foot 5th wheel needs 3 roof ac units. I've never seen that large of an RV with a single unit, but I suppose one may have been removed. Even a 30 foot 5th wheel can struggle with a single AC.
The next thing you need to understand is how your electric system works. Your RV has a 240Vac 50 amp split phase system. This means a maximum of 12,000 watts. Small RV's have a 120Vac 30 amp system that is capable of only 3600 watts. If you connect your RV into a 120Vac 30 amp breaker, it will only have 3600 watts available. That is not enough to run a rooftop AC and a portable AC when the compressors are started. If you are trying to run other electrical devices, it will be impossible.
Another thing to look at is how they ran the electric. Sometimes they do crazy things. My "living room", kitchen and basement outlets are all on a single 15 amp breaker. This means that I can't run a coffee maker and a instapot at the same time. The bathroom and bedroom share another 15 amp circuit. The kitchen should have had at least a dedicated 20 amp circuit. It would have been better with a 20 amp circuit for the kitchen, and another 20 amp circuit for an extra kitchen outlet, the living room, bathroom, and bedroom. I'm good enough with electrical that I am resolving that issue.
It is important to understand how much power devices need. Your microwave should be on it's own 15 amp circuit and will use up to 1500 watts. A 15k btu AC will use about 2000 watts while running on high, but likely over 3000 watts when the compressor starts (for a few seconds). Kitchen appliances, hair dryers and other hair appliances, and electric heaters use over a thousand watts each. Most hair dryers are 1875 watts (try to find a "travel" 1500 watt hair dryer). Coffee makers can use 700-1000 watts. The electric water heater uses 1440 watts while running.
Even smaller devices, like your TV, water pump, fans, lights, and refrigerator all quickly add up in wattage.
Portable AC units are not a good solution unless you are willing to deal with an ugly solution. You want to get a portable AC unit that has 2 air handler hoses. One hose is an air intake, and the other one is the hot air exhaust. AC units create a huge amount of heat in order to cool, in fact that is how a heat pump works. If you do not properly vent that hot air, you will actually heat up the room. This means that you have to run a 3-4 inch hose from the AC to the outside. You also need to properly insulate the hot air exhaust because the vent hose gets very hot (sometimes 110-120F). If you do not insulate it, then that hot hose will radiate hot air back into your RV.
If you are living in the RV full time and not moving it, I would recommend that you buy a mini-split heat pump. For that size RV you will likely need at least an 18K btu unit. I would get a 24K btu unit so that I could hopefully rarely have to run the very low efficiency roof AC.
With a bit of work, you can mount the mini-split compressor side outside the RV (they are designed to be outside). Then configure the air handlers inside of a well insulated wooden box at window level. This will keep the large and ugly air handler unit(s) out of your RV, but the cool air (or heated air) will blow into the RV through an open window. Most mini-split AC units include remote control(s) so that you can change settings from inside the RV.
If it is a more permanent need and/or you need to travel during the hot season, you may be able to find a way to install a smaller mini-split. You will want to get help picking a unit, finding a spot to install it (the compressor unit is often too tall for a 5th wheel roof), and run the cooling lines in side your 5th wheel. You will need to have a spot on the wall to mount the air handler and it will require a dedicated 15-20 amp circuit (depending on size).
You will also want to hire an electrician to install an RV pedestal, like you see at campgrounds. At the minimum you want a single 240Vac 50 amp breaker and outlet. If you will be using the outside mounted minisplit you may want to install a separate 120Vac 20 amp or 30 amp circuit for outside devices. This may require that your workshop electrical outlet be upgraded. I can't say for sure because I don't know what sized breaker box is there nor what equipment it powers. I can say that if it has a 240Vac split phase 100 amp breaker box, it will struggle to handle the 240Vac split phase 50 amp RV, plus enough power to cool the workshop (likely a 240Vac 30 amp circuit to the AC unit) and power tools.
If you are in an area of high temperatures for many months, it is important that you build an RV port (a larger carport). This will create shade so that your RV doesn't heat up as much. That roof should be painted white, preferably special paint that will actually reduce the temperature.
With air temps in the 90F range, your RV roof can easily exceed 120F if it is in the direct sunlight. You may want to put radiant barrier on your roof. Make sure that you close blinds and put insulation in the bathroom skylight. Many people will cut radiant barrier to fit the windows to help keep the inside temperatures down. Installing an insulated skirting can also help, this will help keep the temperature under the 5th wheel down. In hot areas, a surprising amount of heat is transferred into the RV from under the RV. Some of this is because under the 5th wheel may be close to air temperatures plus reflected radiant heating.
Some people will also insulate everything they can on the outside of the RV. They use radiant barrier and/or pool noodles. Slide-outs have next to no insulation. Air can easily seep into the RV at the slide out and where the slide-out mechanism enters the RV.
Radiant barrier, especially in the amount you will need is not cheap. It is much cheaper than the electric work needed to have proper 240Vac 50 amp to your RV and cheaper than buying 2 more RV roof AC units. If you don't want to permanently mount it on the roof, you can tape it down. This is really effective for the slide out roof and side walls (many RV's have their slide out walls painted black, which draws heat in).
I have added insulation to my basement area. I got Harbor Freight rubber "fatigue" matts and installed them on the floor of the basement. This helps with insulation and makes it easier to move around in the basement. The slam doors should have radiant barrier added to it, along with the basement roof and walls.
Every bit of heat that you keep out of the RV is that much less cooling that you need. I doubt that you can reasonably add enough insulation to a 40 foot 5th wheel in order to keep it cool with a single roof AC unit, especially with 90+F outside air temperatures.
A final option, for a long term 5th wheel that will not be moved is to have a geothermal passive cooling system installed. This is basically 4-5 inch tubing buried about 6 foot down (the exact depth is location based). How many tubes and how long (expect them to be several hundred feet long each) they need will be dependent on your location and other factors. Depending on location, this can keep your RV as low as the mid 60F. A decent system will be able to keep it cooled to at least 75F during 100F ambient air temperatures without using any power.
The water pressure issue has already had good things for you to check.
